On systems where many CPUs share one LLC, unbound workqueues using WQ_AFFN_CACHE collapse to a single worker pool, causing heavy spinlock contention on pool->lock. For example, Chuck Lever measured 39% of cycles lost to native_queued_spin_lock_slowpath on a 12-core shared-L3 NFS-over-RDMA system. The existing affinity hierarchy (cpu, smt, cache, numa, system) offers no intermediate option between per-LLC and per-SMT-core granularity. Add WQ_AFFN_CACHE_SHARD, which subdivides each LLC into groups of at most wq_cache_shard_size cores (default 8, tunable via boot parameter). Shards are always split on core (SMT group) boundaries so that Hyper-Threading siblings are never placed in different pods. Cores are distributed across shards as evenly as possible -- for example, 36 cores in a single LLC with max shard size 8 produces 5 shards of 8+7+7+7+7 cores. The implementation follows the same comparator pattern as other affinity scopes: cpu_cache_shard_id() computes a per-CPU shard index on the fly from the already-initialized WQ_AFFN_CACHE and WQ_AFFN_SMT topology, and cpus_share_cache_shard() is passed to init_pod_type(). Benchmark on NVIDIA Grace (72 CPUs, single LLC, 50k items/thread), show cache_shard delivers ~5x the throughput and ~6.5x lower p50 latency compared to cache scope on this 72-core single-LLC system. Suggested-by: Tejun Heo Signed-off-by: Breno Leitao --- include/linux/workqueue.h | 1 + kernel/workqueue.c | 108 ++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 109 insertions(+) diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h index 17543aec2a6e1..50bdb7e30d35f 100644 --- a/include/linux/workqueue.h +++ b/include/linux/workqueue.h @@ -133,6 +133,7 @@ enum wq_affn_scope { WQ_AFFN_CPU, /* one pod per CPU */ WQ_AFFN_SMT, /* one pod per SMT */ WQ_AFFN_CACHE, /* one pod per LLC */ + WQ_AFFN_CACHE_SHARD, /* synthetic sub-LLC shards */ WQ_AFFN_NUMA, /* one pod per NUMA node */ WQ_AFFN_SYSTEM, /* one pod across the whole system */ diff --git a/kernel/workqueue.c b/kernel/workqueue.c index a050971393f1f..ebbc7971b4fa6 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -409,6 +409,7 @@ static const char * const wq_affn_names[WQ_AFFN_NR_TYPES] = { [WQ_AFFN_CPU] = "cpu", [WQ_AFFN_SMT] = "smt", [WQ_AFFN_CACHE] = "cache", + [WQ_AFFN_CACHE_SHARD] = "cache_shard", [WQ_AFFN_NUMA] = "numa", [WQ_AFFN_SYSTEM] = "system", }; @@ -431,6 +432,9 @@ module_param_named(cpu_intensive_warning_thresh, wq_cpu_intensive_warning_thresh static bool wq_power_efficient = IS_ENABLED(CONFIG_WQ_POWER_EFFICIENT_DEFAULT); module_param_named(power_efficient, wq_power_efficient, bool, 0444); +static unsigned int wq_cache_shard_size = 8; +module_param_named(cache_shard_size, wq_cache_shard_size, uint, 0444); + static bool wq_online; /* can kworkers be created yet? */ static bool wq_topo_initialized __read_mostly = false; @@ -8107,6 +8111,104 @@ static bool __init cpus_share_numa(int cpu0, int cpu1) return cpu_to_node(cpu0) == cpu_to_node(cpu1); } +/** + * llc_count_cores - count distinct cores (SMT groups) within a cpumask + * @pod_cpus: the cpumask to scan (typically an LLC pod) + * @smt_pt: the SMT pod type, used to identify sibling groups + * + * A core is represented by the lowest-numbered CPU in its SMT group. Returns + * the number of distinct cores found in @pod_cpus. + */ +static int __init llc_count_cores(const struct cpumask *pod_cpus, + struct wq_pod_type *smt_pt) +{ + const struct cpumask *smt_cpus; + int nr_cores = 0, c; + + for_each_cpu(c, pod_cpus) { + smt_cpus = smt_pt->pod_cpus[smt_pt->cpu_pod[c]]; + if (cpumask_first(smt_cpus) == c) + nr_cores++; + } + + return nr_cores; +} + +/** + * llc_cpu_core_pos - find a CPU's core position within a cpumask + * @cpu: the CPU to locate + * @pod_cpus: the cpumask to scan (typically an LLC pod) + * @smt_pt: the SMT pod type, used to identify sibling groups + * + * Returns the zero-based index of @cpu's core among the distinct cores in + * @pod_cpus, ordered by lowest CPU number in each SMT group. + */ +static int __init llc_cpu_core_pos(int cpu, const struct cpumask *pod_cpus, + struct wq_pod_type *smt_pt) +{ + const struct cpumask *smt_cpus; + int core_pos = 0, c; + + for_each_cpu(c, pod_cpus) { + smt_cpus = smt_pt->pod_cpus[smt_pt->cpu_pod[c]]; + if (cpumask_test_cpu(cpu, smt_cpus)) + break; + if (cpumask_first(smt_cpus) == c) + core_pos++; + } + + return core_pos; +} + +/** + * cpu_cache_shard_id - compute the shard index for a CPU within its LLC pod + * @cpu: the CPU to look up + * + * Returns a shard index that is unique within the CPU's LLC pod. The LLC is + * divided into shards of at most wq_cache_shard_size cores, always split on + * core (SMT group) boundaries so that SMT siblings are never placed in + * different shards. Cores are distributed across shards as evenly as possible. + * + * Example: 36 cores with wq_cache_shard_size=8 gives 5 shards of + * 8+7+7+7+7 cores. + */ +static int __init cpu_cache_shard_id(int cpu) +{ + struct wq_pod_type *cache_pt = &wq_pod_types[WQ_AFFN_CACHE]; + struct wq_pod_type *smt_pt = &wq_pod_types[WQ_AFFN_SMT]; + const struct cpumask *pod_cpus; + int nr_cores, nr_shards, cores_per_shard, remainder, core_pos; + + /* CPUs in the same LLC as @cpu */ + pod_cpus = cache_pt->pod_cpus[cache_pt->cpu_pod[cpu]]; + nr_cores = llc_count_cores(pod_cpus, smt_pt); + + /* Compute number of shards from the max cores per shard */ + nr_shards = DIV_ROUND_UP(nr_cores, wq_cache_shard_size); + /* Distribute cores as evenly as possible across shards */ + cores_per_shard = nr_cores / nr_shards; + remainder = nr_cores % nr_shards; + + core_pos = llc_cpu_core_pos(cpu, pod_cpus, smt_pt); + + /* + * Map core position to shard index. The first @remainder shards have + * (cores_per_shard + 1) cores, the rest have @cores_per_shard cores. + */ + if (core_pos < remainder * (cores_per_shard + 1)) + return core_pos / (cores_per_shard + 1); + + return remainder + (core_pos - remainder * (cores_per_shard + 1)) / cores_per_shard; +} + +static bool __init cpus_share_cache_shard(int cpu0, int cpu1) +{ + if (!cpus_share_cache(cpu0, cpu1)) + return false; + + return cpu_cache_shard_id(cpu0) == cpu_cache_shard_id(cpu1); +} + /** * workqueue_init_topology - initialize CPU pods for unbound workqueues * @@ -8119,9 +8221,15 @@ void __init workqueue_init_topology(void) struct workqueue_struct *wq; int cpu; + if (!wq_cache_shard_size) { + pr_warn("workqueue: cache_shard_size must be > 0, setting to 1\n"); + wq_cache_shard_size = 1; + } + init_pod_type(&wq_pod_types[WQ_AFFN_CPU], cpus_dont_share); init_pod_type(&wq_pod_types[WQ_AFFN_SMT], cpus_share_smt); init_pod_type(&wq_pod_types[WQ_AFFN_CACHE], cpus_share_cache); + init_pod_type(&wq_pod_types[WQ_AFFN_CACHE_SHARD], cpus_share_cache_shard); init_pod_type(&wq_pod_types[WQ_AFFN_NUMA], cpus_share_numa); wq_topo_initialized = true; -- 2.52.0