* Motivation The purpose of this hierarchical split-counter scheme is to: - Minimize contention when incrementing and decrementing counters, - Provide fast access to a sum approximation, - Provide a sum approximation with an acceptable accuracy level when scaling to many-core systems. - Provide approximate and precise comparison of two counters, and between a counter and a value. It aims at fixing the per-mm RSS tracking which has become too inaccurate for OOM killer purposes on large many-core systems [1]. * Design The hierarchical per-CPU counters propagate a sum approximation through a N-way tree. When reaching the batch size, the carry is propagated through a binary tree which consists of logN(nr_cpu_ids) levels. The batch size for each level is twice the batch size of the prior level. Example propagation diagram with 8 cpus through a binary tree: Level 0: 0 1 2 3 4 5 6 7 | / | / | / | / | / | / | / | / | / | / | / | / Level 1: 0 1 2 3 | / | / | / | / | / | / Level 2: 0 1 | / | / | / Level 3: 0 For a binary tree, the maximum inaccuracy is bound by: batch_size * log2(nr_cpus) * nr_cpus which evolves with O(n*log(n)) as the number of CPUs increases. For a N-way tree, the maximum inaccuracy can be pre-calculated based on the the N-arity of each level and the batch size. Link: https://lore.kernel.org/lkml/20250331223516.7810-2-sweettea-kernel@dorminy.me/ # [1] Signed-off-by: Mathieu Desnoyers Cc: Andrew Morton Cc: "Paul E. McKenney" Cc: Steven Rostedt Cc: Masami Hiramatsu Cc: Mathieu Desnoyers Cc: Dennis Zhou Cc: Tejun Heo Cc: Christoph Lameter Cc: Martin Liu Cc: David Rientjes Cc: christian.koenig@amd.com Cc: Shakeel Butt Cc: SeongJae Park Cc: Michal Hocko Cc: Johannes Weiner Cc: Sweet Tea Dorminy Cc: Lorenzo Stoakes Cc: "Liam R . Howlett" Cc: Mike Rapoport Cc: Suren Baghdasaryan Cc: Vlastimil Babka Cc: Christian Brauner Cc: Wei Yang Cc: David Hildenbrand Cc: Miaohe Lin Cc: Al Viro Cc: linux-mm@kvack.org Cc: linux-trace-kernel@vger.kernel.org Cc: Yu Zhao Cc: Roman Gushchin Cc: Mateusz Guzik Cc: Matthew Wilcox Cc: Baolin Wang Cc: Aboorva Devarajan --- Changes since v10: - Fold "mm: Take into account hierarchical percpu tree items for static mm_struct definitions". Changes since v9: - Introduce kerneldoc documentation. - Document structure fields. - Remove inline from percpu_counter_tree_add(). - Reject batch_size==1 which makes no sense. - Fix copy-paste bug in percpu_counter_tree_precise_compare() (wrong inequality comparison). - Rename "inaccuracy" to "accuracy", which makes it easier to document. - Track accuracy limits more precisely. In two's complement signed integers, the range before a n-bit underflow is one unit larger than the range before a n-bit overflow. This sums for all the counters within the tree. Therefore the "under" vs "over" accuracy is not symmetrical. Changes since v8: - Remove migrate guard from the fast path. It does not matter through which path the carry is propagated up the tree. - Rebase on top of v6.18-rc6. - Introduce percpu_counter_tree_init_many and percpu_counter_tree_destroy_many APIs. - Move tree items allocation to the caller. - Introduce percpu_counter_tree_items_size(). - Move percpu_counter_tree_subsystem_init() call before mm_core_init() so percpu_counter_tree_items_size() is initialized before it is used. Changes since v7: - Explicitly initialize the subsystem from start_kernel() right after mm_core_init() so it is up and running before the creation of the first mm at boot. - Remove the module.h include which is not needed with the explicit initialization. - Only consider levels>0 items for order={0,1} nr_items. No functional change except to allocate only the amount of memory which is strictly needed. - Introduce positive precise sum API to handle a scenario where an unlucky precise sum iteration would hit negative counter values concurrently with counter updates. Changes since v5: - Introduce percpu_counter_tree_approximate_sum_positive. - Introduce !CONFIG_SMP static inlines for UP build. - Remove percpu_counter_tree_set_bias from the public API and make it static. Changes since v3: - Add gfp flags to init function. Changes since v2: - Introduce N-way tree to reduce tree depth on larger systems. Changes since v1: - Remove percpu_counter_tree_precise_sum_unbiased from public header, make this function static, - Introduce precise and approximate comparisons between two counters, - Reorder the struct percpu_counter_tree fields, - Introduce approx_sum field, which points to the approximate sum for the percpu_counter_tree_approximate_sum() fast path. --- include/linux/mm_types.h | 6 +- include/linux/percpu_counter_tree.h | 293 ++++++++++++ init/main.c | 2 + lib/Makefile | 1 + lib/percpu_counter_tree.c | 705 ++++++++++++++++++++++++++++ 5 files changed, 1004 insertions(+), 3 deletions(-) create mode 100644 include/linux/percpu_counter_tree.h create mode 100644 lib/percpu_counter_tree.c diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index aa4639888f89..9f861ceabe61 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -1366,9 +1366,9 @@ static inline void __mm_flags_set_mask_bits_word(struct mm_struct *mm, MT_FLAGS_USE_RCU) extern struct mm_struct init_mm; -#define MM_STRUCT_FLEXIBLE_ARRAY_INIT \ -{ \ - [0 ... sizeof(cpumask_t) + MM_CID_STATIC_SIZE - 1] = 0 \ +#define MM_STRUCT_FLEXIBLE_ARRAY_INIT \ +{ \ + [0 ... sizeof(cpumask_t) + MM_CID_STATIC_SIZE + PERCPU_COUNTER_TREE_ITEMS_STATIC_SIZE - 1] = 0 \ } /* Pointer magic because the dynamic array size confuses some compilers. */ diff --git a/include/linux/percpu_counter_tree.h b/include/linux/percpu_counter_tree.h new file mode 100644 index 000000000000..2e8b1ce5cd13 --- /dev/null +++ b/include/linux/percpu_counter_tree.h @@ -0,0 +1,293 @@ +/* SPDX-License-Identifier: GPL-2.0+ OR MIT */ +/* SPDX-FileCopyrightText: 2025 Mathieu Desnoyers */ + +#ifndef _PERCPU_COUNTER_TREE_H +#define _PERCPU_COUNTER_TREE_H + +#include +#include +#include + +#ifdef CONFIG_SMP + +#if NR_CPUS == (1U << 0) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 0 +#elif NR_CPUS <= (1U << 1) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 1 +#elif NR_CPUS <= (1U << 2) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 3 +#elif NR_CPUS <= (1U << 3) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 7 +#elif NR_CPUS <= (1U << 4) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 7 +#elif NR_CPUS <= (1U << 5) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 11 +#elif NR_CPUS <= (1U << 6) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 21 +#elif NR_CPUS <= (1U << 7) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 21 +#elif NR_CPUS <= (1U << 8) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 37 +#elif NR_CPUS <= (1U << 9) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 73 +#elif NR_CPUS <= (1U << 10) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 149 +#elif NR_CPUS <= (1U << 11) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 293 +#elif NR_CPUS <= (1U << 12) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 585 +#elif NR_CPUS <= (1U << 13) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 1173 +#elif NR_CPUS <= (1U << 14) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 2341 +#elif NR_CPUS <= (1U << 15) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 4681 +#elif NR_CPUS <= (1U << 16) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 4681 +#elif NR_CPUS <= (1U << 17) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 8777 +#elif NR_CPUS <= (1U << 18) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 17481 +#elif NR_CPUS <= (1U << 19) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 34953 +#elif NR_CPUS <= (1U << 20) +# define PERCPU_COUNTER_TREE_STATIC_NR_ITEMS 69905 +#else +# error "Unsupported number of CPUs." +#endif + +struct percpu_counter_tree_level_item { + atomic_t count; /* + * Count the number of carry fort this tree item. + * The carry counter is kept at the order of the + * carry accounted for at this tree level. + */ +} ____cacheline_aligned_in_smp; + +#define PERCPU_COUNTER_TREE_ITEMS_STATIC_SIZE \ + (PERCPU_COUNTER_TREE_STATIC_NR_ITEMS * sizeof(struct percpu_counter_tree_level_item)) + +struct percpu_counter_tree { + /* Fast-path fields. */ + unsigned int __percpu *level0; /* Pointer to per-CPU split counters (tree level 0). */ + unsigned int level0_bit_mask; /* Bit mask to apply to detect carry propagation from tree level 0. */ + union { + unsigned int *i; /* Approximate sum for single-CPU topology. */ + atomic_t *a; /* Approximate sum for SMP topology. */ + } approx_sum; + int bias; /* Bias to apply to counter precise and approximate values. */ + + /* Slow-path fields. */ + struct percpu_counter_tree_level_item *items; /* Array of tree items for levels 1 to N. */ + unsigned int batch_size; /* + * The batch size is the increment step at level 0 which + * triggers a carry propagation. The batch size is required + * to be greater than 1, and a power of 2. + */ + /* + * The tree approximate sum is guaranteed to be within this accuracy range: + * (precise_sum - approx_accuracy_range.under) <= approx_sum <= (precise_sum + approx_accuracy_range.over). + * This accuracy is derived from the hardware topology and the tree batch_size. + * The "under" accuracy is larger than the "over" accuracy because the negative range of a + * two's complement signed integer is one unit larger than the positive range. This delta + * is summed for each tree item, which leads to a significantly larger "under" accuracy range + * compared to the "over" accuracy range. + */ + struct { + unsigned int under; + unsigned int over; + } approx_accuracy_range; +}; + +size_t percpu_counter_tree_items_size(void); +int percpu_counter_tree_init_many(struct percpu_counter_tree *counters, struct percpu_counter_tree_level_item *items, + unsigned int nr_counters, unsigned int batch_size, gfp_t gfp_flags); +int percpu_counter_tree_init(struct percpu_counter_tree *counter, struct percpu_counter_tree_level_item *items, + unsigned int batch_size, gfp_t gfp_flags); +void percpu_counter_tree_destroy_many(struct percpu_counter_tree *counter, unsigned int nr_counters); +void percpu_counter_tree_destroy(struct percpu_counter_tree *counter); +void percpu_counter_tree_add(struct percpu_counter_tree *counter, int inc); +int percpu_counter_tree_precise_sum(struct percpu_counter_tree *counter); +int percpu_counter_tree_approximate_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b); +int percpu_counter_tree_approximate_compare_value(struct percpu_counter_tree *counter, int v); +int percpu_counter_tree_precise_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b); +int percpu_counter_tree_precise_compare_value(struct percpu_counter_tree *counter, int v); +void percpu_counter_tree_set(struct percpu_counter_tree *counter, int v); +void percpu_counter_tree_approximate_accuracy_range(struct percpu_counter_tree *counter, + unsigned int *under, unsigned int *over); +int percpu_counter_tree_subsystem_init(void); + +/** + * percpu_counter_tree_approximate_sum() - Return approximate counter sum. + * @counter: The counter to sum. + * + * Querying the approximate sum is fast, but it is only accurate within + * the bounds delimited by percpu_counter_tree_approximate_accuracy_range(). + * This is meant to be used when speed is preferred over accuracy. + * + * Return: The current approximate counter sum. + */ +static inline +int percpu_counter_tree_approximate_sum(struct percpu_counter_tree *counter) +{ + unsigned int v; + + if (!counter->level0_bit_mask) + v = READ_ONCE(*counter->approx_sum.i); + else + v = atomic_read(counter->approx_sum.a); + return (int) (v + (unsigned int)READ_ONCE(counter->bias)); +} + +#else /* !CONFIG_SMP */ + +#define PERCPU_COUNTER_TREE_ITEMS_STATIC_SIZE 0 + +struct percpu_counter_tree_level_item; + +struct percpu_counter_tree { + atomic_t count; +}; + +static inline +size_t percpu_counter_tree_items_size(void) +{ + return 0; +} + +static inline +int percpu_counter_tree_init_many(struct percpu_counter_tree *counters, struct percpu_counter_tree_level_item *items, + unsigned int nr_counters, unsigned int batch_size, gfp_t gfp_flags) +{ + for (unsigned int i = 0; i < nr_counters; i++) + atomic_set(&counters[i].count, 0); + return 0; +} + +static inline +int percpu_counter_tree_init(struct percpu_counter_tree *counter, struct percpu_counter_tree_level_item *items, + unsigned int batch_size, gfp_t gfp_flags) +{ + return percpu_counter_tree_init_many(counter, items, 1, batch_size, gfp_flags); +} + +static inline +void percpu_counter_tree_destroy_many(struct percpu_counter_tree *counter, unsigned int nr_counters) +{ +} + +static inline +void percpu_counter_tree_destroy(struct percpu_counter_tree *counter) +{ +} + +static inline +int percpu_counter_tree_precise_sum(struct percpu_counter_tree *counter) +{ + return atomic_read(&counter->count); +} + +static inline +int percpu_counter_tree_precise_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b) +{ + int count_a = percpu_counter_tree_precise_sum(a), + count_b = percpu_counter_tree_precise_sum(b); + + if (count_a == count_b) + return 0; + if (count_a < count_b) + return -1; + return 1; +} + +static inline +int percpu_counter_tree_precise_compare_value(struct percpu_counter_tree *counter, int v) +{ + int count = percpu_counter_tree_precise_sum(counter); + + if (count == v) + return 0; + if (count < v) + return -1; + return 1; +} + +static inline +int percpu_counter_tree_approximate_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b) +{ + return percpu_counter_tree_precise_compare(a, b); +} + +static inline +int percpu_counter_tree_approximate_compare_value(struct percpu_counter_tree *counter, int v) +{ + return percpu_counter_tree_precise_compare_value(counter, v); +} + +static inline +void percpu_counter_tree_set(struct percpu_counter_tree *counter, int v) +{ + atomic_set(&counter->count, v); +} + +static inline +void percpu_counter_tree_approximate_accuracy_range(struct percpu_counter_tree *counter, + unsigned int *under, unsigned int *over) +{ + *under = 0; + *over = 0; +} + +static inline +void percpu_counter_tree_add(struct percpu_counter_tree *counter, int inc) +{ + atomic_add(inc, &counter->count); +} + +static inline +int percpu_counter_tree_approximate_sum(struct percpu_counter_tree *counter) +{ + return percpu_counter_tree_precise_sum(counter); +} + +static inline +int percpu_counter_tree_subsystem_init(void) +{ + return 0; +} + +#endif /* CONFIG_SMP */ + +/** + * percpu_counter_tree_approximate_sum_positive() - Return a positive approximate counter sum. + * @counter: The counter to sum. + * + * Return an approximate counter sum which is guaranteed to be greater + * or equal to 0. + * + * Return: The current positive approximate counter sum. + */ +static inline +int percpu_counter_tree_approximate_sum_positive(struct percpu_counter_tree *counter) +{ + int v = percpu_counter_tree_approximate_sum(counter); + return v > 0 ? v : 0; +} + +/** + * percpu_counter_tree_precise_sum_positive() - Return a positive precise counter sum. + * @counter: The counter to sum. + * + * Return a precise counter sum which is guaranteed to be greater + * or equal to 0. + * + * Return: The current positive precise counter sum. + */ +static inline +int percpu_counter_tree_precise_sum_positive(struct percpu_counter_tree *counter) +{ + int v = percpu_counter_tree_precise_sum(counter); + return v > 0 ? v : 0; +} + +#endif /* _PERCPU_COUNTER_TREE_H */ diff --git a/init/main.c b/init/main.c index b84818ad9685..85bcc7c1ccd0 100644 --- a/init/main.c +++ b/init/main.c @@ -104,6 +104,7 @@ #include #include #include +#include #include #include @@ -1063,6 +1064,7 @@ void start_kernel(void) vfs_caches_init_early(); sort_main_extable(); trap_init(); + percpu_counter_tree_subsystem_init(); mm_core_init(); maple_tree_init(); poking_init(); diff --git a/lib/Makefile b/lib/Makefile index aaf677cf4527..bff117556424 100644 --- a/lib/Makefile +++ b/lib/Makefile @@ -181,6 +181,7 @@ obj-$(CONFIG_TEXTSEARCH_KMP) += ts_kmp.o obj-$(CONFIG_TEXTSEARCH_BM) += ts_bm.o obj-$(CONFIG_TEXTSEARCH_FSM) += ts_fsm.o obj-$(CONFIG_SMP) += percpu_counter.o +obj-$(CONFIG_SMP) += percpu_counter_tree.o obj-$(CONFIG_AUDIT_GENERIC) += audit.o obj-$(CONFIG_AUDIT_COMPAT_GENERIC) += compat_audit.o diff --git a/lib/percpu_counter_tree.c b/lib/percpu_counter_tree.c new file mode 100644 index 000000000000..9b7ae7df5530 --- /dev/null +++ b/lib/percpu_counter_tree.c @@ -0,0 +1,705 @@ +// SPDX-License-Identifier: GPL-2.0+ OR MIT +// SPDX-FileCopyrightText: 2025 Mathieu Desnoyers + +/* + * Split Counters With Tree Approximation Propagation + * + * * Propagation diagram when reaching batch size thresholds (± batch size): + * + * Example diagram for 8 CPUs: + * + * log2(8) = 3 levels + * + * At each level, each pair propagates its values to the next level when + * reaching the batch size thresholds. + * + * Counters at levels 0, 1, 2 can be kept on a single byte ([-128 .. +127] range), + * although it may be relevant to keep them on 32-bit counters for + * simplicity. (complexity vs memory footprint tradeoff) + * + * Counter at level 3 can be kept on a 32-bit counter. + * + * Level 0: 0 1 2 3 4 5 6 7 + * | / | / | / | / + * | / | / | / | / + * | / | / | / | / + * Level 1: 0 1 2 3 + * | / | / + * | / | / + * | / | / + * Level 2: 0 1 + * | / + * | / + * | / + * Level 3: 0 + * + * * Approximation accuracy: + * + * BATCH(level N): Level N batch size. + * + * Example for BATCH(level 0) = 32. + * + * BATCH(level 0) = 32 + * BATCH(level 1) = 64 + * BATCH(level 2) = 128 + * BATCH(level N) = BATCH(level 0) * 2^N + * + * per-counter global + * accuracy accuracy + * Level 0: [ -32 .. +31] ±256 (8 * 32) + * Level 1: [ -64 .. +63] ±256 (4 * 64) + * Level 2: [-128 .. +127] ±256 (2 * 128) + * Total: ------ ±768 (log2(nr_cpu_ids) * BATCH(level 0) * nr_cpu_ids) + * + * Note that the global accuracy can be calculated more precisely + * by taking into account that the positive accuracy range is + * 31 rather than 32. + * + * ----- + * + * Approximate Sum Carry Propagation + * + * Let's define a number of counter bits for each level, e.g.: + * + * log2(BATCH(level 0)) = log2(32) = 5 + * + * nr_bit value_mask range + * Level 0: 5 bits v 0 .. +31 + * Level 1: 1 bit (v & ~((1UL << 5) - 1)) 0 .. +63 + * Level 2: 1 bit (v & ~((1UL << 6) - 1)) 0 .. +127 + * Level 3: 25 bits (v & ~((1UL << 7) - 1)) 0 .. 2^32-1 + * + * Note: Use a full 32-bit per-cpu counter at level 0 to allow precise sum. + * + * Note: Use cacheline aligned counters at levels above 0 to prevent false sharing. + * If memory footprint is an issue, a specialized allocator could be used + * to eliminate padding. + * + * Example with expanded values: + * + * counter_add(counter, inc): + * + * if (!inc) + * return; + * + * res = percpu_add_return(counter @ Level 0, inc); + * orig = res - inc; + * if (inc < 0) { + * inc = -(-inc & ~0b00011111); // Clear used bits + * // xor bit 5: underflow + * if ((inc ^ orig ^ res) & 0b00100000) + * inc -= 0b00100000; + * } else { + * inc &= ~0b00011111; // Clear used bits + * // xor bit 5: overflow + * if ((inc ^ orig ^ res) & 0b00100000) + * inc += 0b00100000; + * } + * if (!inc) + * return; + * + * res = atomic_add_return(counter @ Level 1, inc); + * orig = res - inc; + * if (inc < 0) { + * inc = -(-inc & ~0b00111111); // Clear used bits + * // xor bit 6: underflow + * if ((inc ^ orig ^ res) & 0b01000000) + * inc -= 0b01000000; + * } else { + * inc &= ~0b00111111; // Clear used bits + * // xor bit 6: overflow + * if ((inc ^ orig ^ res) & 0b01000000) + * inc += 0b01000000; + * } + * if (!inc) + * return; + * + * res = atomic_add_return(counter @ Level 2, inc); + * orig = res - inc; + * if (inc < 0) { + * inc = -(-inc & ~0b01111111); // Clear used bits + * // xor bit 7: underflow + * if ((inc ^ orig ^ res) & 0b10000000) + * inc -= 0b10000000; + * } else { + * inc &= ~0b01111111; // Clear used bits + * // xor bit 7: overflow + * if ((inc ^ orig ^ res) & 0b10000000) + * inc += 0b10000000; + * } + * if (!inc) + * return; + * + * atomic_add(counter @ Level 3, inc); + */ + +#include +#include +#include +#include +#include +#include +#include + +#define MAX_NR_LEVELS 5 + +/* + * The counter configuration is selected at boot time based on the + * hardware topology. + */ +struct counter_config { + unsigned int nr_items; /* + * nr_items is the number of items in the tree for levels 1 + * up to and including the final level (approximate sum). + * It excludes the level 0 per-CPU counters. + */ + unsigned char nr_levels; /* + * nr_levels is the number of hierarchical counter tree levels. + * It excludes the final level (approximate sum). + */ + unsigned char n_arity_order[MAX_NR_LEVELS]; /* + * n-arity of tree nodes for each level from + * 0 to (nr_levels - 1). + */ +}; + +static const struct counter_config per_nr_cpu_order_config[] = { + [0] = { .nr_items = 0, .nr_levels = 0, .n_arity_order = { 0 } }, + [1] = { .nr_items = 1, .nr_levels = 1, .n_arity_order = { 1 } }, + [2] = { .nr_items = 3, .nr_levels = 2, .n_arity_order = { 1, 1 } }, + [3] = { .nr_items = 7, .nr_levels = 3, .n_arity_order = { 1, 1, 1 } }, + [4] = { .nr_items = 7, .nr_levels = 3, .n_arity_order = { 2, 1, 1 } }, + [5] = { .nr_items = 11, .nr_levels = 3, .n_arity_order = { 2, 2, 1 } }, + [6] = { .nr_items = 21, .nr_levels = 3, .n_arity_order = { 2, 2, 2 } }, + [7] = { .nr_items = 21, .nr_levels = 3, .n_arity_order = { 3, 2, 2 } }, + [8] = { .nr_items = 37, .nr_levels = 3, .n_arity_order = { 3, 3, 2 } }, + [9] = { .nr_items = 73, .nr_levels = 3, .n_arity_order = { 3, 3, 3 } }, + [10] = { .nr_items = 149, .nr_levels = 4, .n_arity_order = { 3, 3, 2, 2 } }, + [11] = { .nr_items = 293, .nr_levels = 4, .n_arity_order = { 3, 3, 3, 2 } }, + [12] = { .nr_items = 585, .nr_levels = 4, .n_arity_order = { 3, 3, 3, 3 } }, + [13] = { .nr_items = 1173, .nr_levels = 5, .n_arity_order = { 3, 3, 3, 2, 2 } }, + [14] = { .nr_items = 2341, .nr_levels = 5, .n_arity_order = { 3, 3, 3, 3, 2 } }, + [15] = { .nr_items = 4681, .nr_levels = 5, .n_arity_order = { 3, 3, 3, 3, 3 } }, + [16] = { .nr_items = 4681, .nr_levels = 5, .n_arity_order = { 4, 3, 3, 3, 3 } }, + [17] = { .nr_items = 8777, .nr_levels = 5, .n_arity_order = { 4, 4, 3, 3, 3 } }, + [18] = { .nr_items = 17481, .nr_levels = 5, .n_arity_order = { 4, 4, 4, 3, 3 } }, + [19] = { .nr_items = 34953, .nr_levels = 5, .n_arity_order = { 4, 4, 4, 4, 3 } }, + [20] = { .nr_items = 69905, .nr_levels = 5, .n_arity_order = { 4, 4, 4, 4, 4 } }, +}; + +static const struct counter_config *counter_config; /* Hierarchical counter configuration for the hardware topology. */ +static unsigned int nr_cpus_order, /* Order of nr_cpu_ids. */ + accuracy_multiplier; /* Calculate accuracy for a given batch size (multiplication factor). */ + +static +int __percpu_counter_tree_init(struct percpu_counter_tree *counter, + unsigned int batch_size, gfp_t gfp_flags, + unsigned int __percpu *level0, + struct percpu_counter_tree_level_item *items) +{ + /* Batch size must be greater than 1, and a power of 2. */ + if (WARN_ON(batch_size <= 1 || (batch_size & (batch_size - 1)))) + return -EINVAL; + counter->batch_size = batch_size; + counter->bias = 0; + counter->level0 = level0; + counter->items = items; + if (!nr_cpus_order) { + counter->approx_sum.i = per_cpu_ptr(counter->level0, 0); + counter->level0_bit_mask = 0; + } else { + counter->approx_sum.a = &counter->items[counter_config->nr_items - 1].count; + counter->level0_bit_mask = 1UL << get_count_order(batch_size); + } + /* + * Each tree item signed integer has a negative range which is + * one unit greater than the positive range. + */ + counter->approx_accuracy_range.under = batch_size * accuracy_multiplier; + counter->approx_accuracy_range.over = (batch_size - 1) * accuracy_multiplier; + return 0; +} + +/** + * percpu_counter_tree_init_many() - Initialize many per-CPU counter trees. + * @counters: An array of @nr_counters counters to initialize. + * Their memory is provided by the caller. + * @items: Pointer to memory area where to store tree items. + * This memory is provided by the caller. + * Its size needs to be at least @nr_counters * percpu_counter_tree_items_size(). + * @nr_counters: The number of counter trees to initialize + * @batch_size: The batch size is the increment step at level 0 which triggers a + * carry propagation. + * The batch size is required to be greater than 1, and a power of 2. + * @gfp_flags: gfp flags to pass to the per-CPU allocator. + * + * Initialize many per-CPU counter trees using a single per-CPU + * allocator invocation for @nr_counters counters. + * + * Return: + * * %0: Success + * * %-EINVAL: - Invalid @batch_size argument + * * %-ENOMEM: - Out of memory + */ +int percpu_counter_tree_init_many(struct percpu_counter_tree *counters, struct percpu_counter_tree_level_item *items, + unsigned int nr_counters, unsigned int batch_size, gfp_t gfp_flags) +{ + void __percpu *level0, *level0_iter; + size_t counter_size, items_size = 0; + void *items_iter; + unsigned int i; + int ret; + + counter_size = ALIGN(sizeof(*counters), __alignof__(*counters)); + level0 = __alloc_percpu_gfp(nr_counters * counter_size, + __alignof__(*counters), gfp_flags); + if (!level0) + return -ENOMEM; + if (nr_cpus_order) { + items_size = percpu_counter_tree_items_size(); + memset(items, 0, items_size * nr_counters); + } + level0_iter = level0; + items_iter = items; + for (i = 0; i < nr_counters; i++) { + ret = __percpu_counter_tree_init(&counters[i], batch_size, gfp_flags, level0_iter, items_iter); + if (ret) + goto free_level0; + level0_iter += counter_size; + if (nr_cpus_order) + items_iter += items_size; + } + return 0; + +free_level0: + free_percpu(level0); + return ret; +} + +/** + * percpu_counter_tree_init() - Initialize one per-CPU counter tree. + * @counter: Counter to initialize. + * Its memory is provided by the caller. + * @items: Pointer to memory area where to store tree items. + * This memory is provided by the caller. + * Its size needs to be at least percpu_counter_tree_items_size(). + * @batch_size: The batch size is the increment step at level 0 which triggers a + * carry propagation. + * The batch size is required to be greater than 1, and a power of 2. + * @gfp_flags: gfp flags to pass to the per-CPU allocator. + * + * Initialize one per-CPU counter tree. + * + * Return: + * * %0: Success + * * %-EINVAL: - Invalid @batch_size argument + * * %-ENOMEM: - Out of memory + */ +int percpu_counter_tree_init(struct percpu_counter_tree *counter, struct percpu_counter_tree_level_item *items, + unsigned int batch_size, gfp_t gfp_flags) +{ + return percpu_counter_tree_init_many(counter, items, 1, batch_size, gfp_flags); +} + +/** + * percpu_counter_tree_destroy_many() - Destroy many per-CPU counter trees. + * @counters: Array of counters trees to destroy. + * @nr_counters: The number of counter trees to destroy. + * + * Release internal resources allocated for @nr_counters per-CPU counter trees. + */ + +void percpu_counter_tree_destroy_many(struct percpu_counter_tree *counters, unsigned int nr_counters) +{ + free_percpu(counters->level0); +} + +/** + * percpu_counter_tree_destroy() - Destroy one per-CPU counter tree. + * @counter: Counter to destroy. + * + * Release internal resources allocated for one per-CPU counter tree. + */ +void percpu_counter_tree_destroy(struct percpu_counter_tree *counter) +{ + return percpu_counter_tree_destroy_many(counter, 1); +} + +static +int percpu_counter_tree_carry(int orig, int res, int inc, unsigned int bit_mask) +{ + if (inc < 0) { + inc = -(-inc & ~(bit_mask - 1)); + /* + * xor bit_mask: underflow. + * + * If inc has bit set, decrement an additional bit if + * there is _no_ bit transition between orig and res. + * Else, inc has bit cleared, decrement an additional + * bit if there is a bit transition between orig and + * res. + */ + if ((inc ^ orig ^ res) & bit_mask) + inc -= bit_mask; + } else { + inc &= ~(bit_mask - 1); + /* + * xor bit_mask: overflow. + * + * If inc has bit set, increment an additional bit if + * there is _no_ bit transition between orig and res. + * Else, inc has bit cleared, increment an additional + * bit if there is a bit transition between orig and + * res. + */ + if ((inc ^ orig ^ res) & bit_mask) + inc += bit_mask; + } + return inc; +} + +/* + * It does not matter through which path the carry propagates up the + * tree, therefore there is no need to disable preemption because the + * cpu number is only used to favor cache locality. + */ +static +void percpu_counter_tree_add_slowpath(struct percpu_counter_tree *counter, int inc) +{ + unsigned int level_items, nr_levels = counter_config->nr_levels, + level, n_arity_order, bit_mask; + struct percpu_counter_tree_level_item *item = counter->items; + unsigned int cpu = raw_smp_processor_id(); + + WARN_ON_ONCE(!nr_cpus_order); /* Should never be called for 1 cpu. */ + + n_arity_order = counter_config->n_arity_order[0]; + bit_mask = counter->level0_bit_mask << n_arity_order; + level_items = 1U << (nr_cpus_order - n_arity_order); + + for (level = 1; level < nr_levels; level++) { + atomic_t *count = &item[cpu & (level_items - 1)].count; + unsigned int orig, res; + + res = atomic_add_return_relaxed(inc, count); + orig = res - inc; + inc = percpu_counter_tree_carry(orig, res, inc, bit_mask); + if (likely(!inc)) + return; + item += level_items; + n_arity_order = counter_config->n_arity_order[level]; + level_items >>= n_arity_order; + bit_mask <<= n_arity_order; + } + atomic_add(inc, counter->approx_sum.a); +} + +/** + * percpu_counter_tree_add() - Add to a per-CPU counter tree. + * @counter: Counter added to. + * @inc: Increment value (either positive or negative). + * + * Add @inc to a per-CPU counter tree. This is a fast-path which will + * typically increment per-CPU counters as long as there is no carry + * greater or equal to the counter tree batch size. + */ +void percpu_counter_tree_add(struct percpu_counter_tree *counter, int inc) +{ + unsigned int bit_mask = counter->level0_bit_mask, orig, res; + + res = this_cpu_add_return(*counter->level0, inc); + orig = res - inc; + inc = percpu_counter_tree_carry(orig, res, inc, bit_mask); + if (likely(!inc)) + return; + percpu_counter_tree_add_slowpath(counter, inc); +} + + +static +int percpu_counter_tree_precise_sum_unbiased(struct percpu_counter_tree *counter) +{ + unsigned int sum = 0; + int cpu; + + for_each_possible_cpu(cpu) + sum += *per_cpu_ptr(counter->level0, cpu); + return (int) sum; +} + +/** + * percpu_counter_tree_precise_sum() - Return precise counter sum. + * @counter: The counter to sum. + * + * Querying the precise sum is relatively expensive because it needs to + * iterate over all CPUs. + * This is meant to be used when accuracy is preferred over speed. + * + * Return: The current precise counter sum. + */ +int percpu_counter_tree_precise_sum(struct percpu_counter_tree *counter) +{ + return percpu_counter_tree_precise_sum_unbiased(counter) + READ_ONCE(counter->bias); +} + +static +int compare_delta(int delta, unsigned int accuracy_neg, unsigned int accuracy_pos) +{ + if (delta >= 0) { + if (delta <= accuracy_pos) + return 0; + else + return 1; + } else { + if (-delta <= accuracy_neg) + return 0; + else + return -1; + } +} + +/** + * percpu_counter_tree_approximate_compare - Approximated comparison of two counter trees. + * @a: First counter to compare. + * @b: Second counter to compare. + * + * Evaluate an approximate comparison of two counter trees. + * This approximation comparison is fast, and provides an accurate + * answer if the counters are found to be either less than or greater + * than the other. However, if the approximated comparison returns + * 0, the counters respective sums are found to be within the two + * counters accuracy range. + * + * Return: + * * %0 - Counters @a and @b do not differ by more than the sum of their respective + * accuracy ranges. + * * %-1 - Counter @a less than counter @b. + * * %1 - Counter @a is greater than counter @b. + */ +int percpu_counter_tree_approximate_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b) +{ + return compare_delta(percpu_counter_tree_approximate_sum(a) - percpu_counter_tree_approximate_sum(b), + a->approx_accuracy_range.over + b->approx_accuracy_range.under, + a->approx_accuracy_range.under + b->approx_accuracy_range.over); +} + +/** + * percpu_counter_tree_approximate_compare_value - Approximated comparison of a counter tree against a given value. + * @counter: Counter to compare. + * @v: Value to compare. + * + * Evaluate an approximate comparison of a counter tree against a given value. + * This approximation comparison is fast, and provides an accurate + * answer if the counter is found to be either less than or greater + * than the value. However, if the approximated comparison returns + * 0, the value is within the counter accuracy range. + * + * Return: + * * %0 - The value @v is within the accuracy range of the counter. + * * %-1 - The value @v is less than the counter. + * * %1 - The value @v is greater than the counter. + */ +int percpu_counter_tree_approximate_compare_value(struct percpu_counter_tree *counter, int v) +{ + return compare_delta(v - percpu_counter_tree_approximate_sum(counter), + counter->approx_accuracy_range.under, + counter->approx_accuracy_range.over); +} + +/** + * percpu_counter_tree_precise_compare - Precise comparison of two counter trees. + * @a: First counter to compare. + * @b: Second counter to compare. + * + * Evaluate a precise comparison of two counter trees. + * As an optimization, it uses the approximate counter comparison + * to quickly compare counters which are far apart. Only cases where + * counter sums are within the accuracy range require precise counter + * sums. + * + * Return: + * * %0 - Counters are equal. + * * %-1 - Counter @a less than counter @b. + * * %1 - Counter @a is greater than counter @b. + */ +int percpu_counter_tree_precise_compare(struct percpu_counter_tree *a, struct percpu_counter_tree *b) +{ + int count_a = percpu_counter_tree_approximate_sum(a), + count_b = percpu_counter_tree_approximate_sum(b); + unsigned int accuracy_a, accuracy_b; + int delta = count_a - count_b; + int res; + + res = compare_delta(delta, + a->approx_accuracy_range.over + b->approx_accuracy_range.under, + a->approx_accuracy_range.under + b->approx_accuracy_range.over); + /* The values are distanced enough for an accurate approximated comparison. */ + if (res) + return res; + + /* + * The approximated comparison is within the accuracy range, therefore at least one + * precise sum is needed. Sum the counter which has the largest accuracy first. + */ + if (delta >= 0) { + accuracy_a = a->approx_accuracy_range.under; + accuracy_b = b->approx_accuracy_range.over; + } else { + accuracy_a = a->approx_accuracy_range.over; + accuracy_b = b->approx_accuracy_range.under; + } + if (accuracy_b < accuracy_a) { + count_a = percpu_counter_tree_precise_sum(a); + res = compare_delta(count_a - count_b, + b->approx_accuracy_range.under, + b->approx_accuracy_range.over); + if (res) + return res; + /* Precise sum of second counter is required. */ + count_b = percpu_counter_tree_precise_sum(b); + } else { + count_b = percpu_counter_tree_precise_sum(b); + res = compare_delta(count_a - count_b, + a->approx_accuracy_range.over, + a->approx_accuracy_range.under); + if (res) + return res; + /* Precise sum of second counter is required. */ + count_a = percpu_counter_tree_precise_sum(a); + } + if (count_a - count_b < 0) + return -1; + if (count_a - count_b > 0) + return 1; + return 0; +} + +/** + * percpu_counter_tree_precise_compare_value - Precise comparison of a counter tree against a given value. + * @counter: Counter to compare. + * @v: Value to compare. + * + * Evaluate a precise comparison of a counter tree against a given value. + * As an optimization, it uses the approximate counter comparison + * to quickly identify whether the counter and value are far apart. + * Only cases where the value is within the counter accuracy range + * require a precise counter sum. + * + * Return: + * * %0 - The value @v is equal to the counter. + * * %-1 - The value @v is less than the counter. + * * %1 - The value @v is greater than the counter. + */ +int percpu_counter_tree_precise_compare_value(struct percpu_counter_tree *counter, int v) +{ + int count = percpu_counter_tree_approximate_sum(counter); + int res; + + res = compare_delta(v - count, + counter->approx_accuracy_range.under, + counter->approx_accuracy_range.over); + /* The values are distanced enough for an accurate approximated comparison. */ + if (res) + return res; + + /* Precise sum is required. */ + count = percpu_counter_tree_precise_sum(counter); + if (v - count < 0) + return -1; + if (v - count > 0) + return 1; + return 0; +} + +static +void percpu_counter_tree_set_bias(struct percpu_counter_tree *counter, int bias) +{ + WRITE_ONCE(counter->bias, bias); +} + +/** + * percpu_counter_tree_set - Set the counter tree sum to a given value. + * @counter: Counter to set. + * @v: Value to set. + * + * Set the counter sum to a given value. It can be useful for instance + * to reset the counter sum to 0. Note that even after setting the + * counter sum to a given value, the counter sum approximation can + * return any value within the accuracy range around that value. + */ +void percpu_counter_tree_set(struct percpu_counter_tree *counter, int v) +{ + percpu_counter_tree_set_bias(counter, + v - percpu_counter_tree_precise_sum_unbiased(counter)); +} + +/** + * percpu_counter_tree_approximate_accuracy_range - Query the accuracy range for a counter tree. + * @counter: Counter to query. + * @under: Pointer where to store the to the accuracy range below the approximation. + * @over: Pointer where to store the to the accuracy range above the approximation. + * + * Query the accuracy range limits for the counter. + * Because of two's complement binary representation, the "under" range is typically + * slightly larger than the "over" range. + * Those values are derived from the hardware topology and the counter tree batch size. + * They are invariant for a given counter tree. + * Using this function should not be typically required, see the following functions instead: + * * percpu_counter_tree_approximate_compare(), + * * percpu_counter_tree_approximate_compare_value(), + * * percpu_counter_tree_precise_compare(), + * * percpu_counter_tree_precise_compare_value(). + */ +void percpu_counter_tree_approximate_accuracy_range(struct percpu_counter_tree *counter, + unsigned int *under, unsigned int *over) +{ + *under = counter->approx_accuracy_range.under; + *over = counter->approx_accuracy_range.over; +} + +/* + * percpu_counter_tree_items_size - Query the size required for counter tree items. + * + * Query the size of the memory area required to hold the counter tree + * items. This depends on the hardware topology and is invariant after + * boot. + * + * Return: Size required to hold tree items. + */ +size_t percpu_counter_tree_items_size(void) +{ + if (!nr_cpus_order) + return 0; + return counter_config->nr_items * sizeof(struct percpu_counter_tree_level_item); +} + +static void __init calculate_accuracy_topology(void) +{ + unsigned int nr_levels = counter_config->nr_levels, level; + unsigned int level_items = 1U << nr_cpus_order; + unsigned int batch_size = 1; + + for (level = 0; level < nr_levels; level++) { + unsigned int n_arity_order = counter_config->n_arity_order[level]; + + /* + * The accuracy multiplier is derived from a batch size of 1 + * to speed up calculating the accuracy at tree initialization. + */ + accuracy_multiplier += batch_size * level_items; + batch_size <<= n_arity_order; + level_items >>= n_arity_order; + } +} + +int __init percpu_counter_tree_subsystem_init(void) +{ + nr_cpus_order = get_count_order(nr_cpu_ids); + if (WARN_ON_ONCE(nr_cpus_order >= ARRAY_SIZE(per_nr_cpu_order_config))) { + printk(KERN_ERR "Unsupported number of CPUs (%u)\n", nr_cpu_ids); + return -1; + } + counter_config = &per_nr_cpu_order_config[nr_cpus_order]; + calculate_accuracy_topology(); + return 0; +} -- 2.39.5