Currently, refresh_cpu_vm_stats returns an int, indicating how many changes were made during its updates. Using this information, callers like vmstat_update can heuristically determine if more work will be done in the future. However, all of refresh_cpu_vm_stats's callers either (a) ignore the result, only caring about performing the updates, or (b) only care about whether changes were made, but not *how many* changes were made. Simplify the code by returning a bool instead to indicate if updates were made. In addition, simplify fold_diff and decay_pcp_high to return a bool for the same reason. Signed-off-by: Joshua Hahn --- include/linux/gfp.h | 2 +- mm/page_alloc.c | 8 ++++---- mm/vmstat.c | 26 +++++++++++++------------- 3 files changed, 18 insertions(+), 18 deletions(-) diff --git a/include/linux/gfp.h b/include/linux/gfp.h index 5ebf26fcdcfa..63c72cb1d117 100644 --- a/include/linux/gfp.h +++ b/include/linux/gfp.h @@ -386,7 +386,7 @@ extern void free_pages(unsigned long addr, unsigned int order); #define free_page(addr) free_pages((addr), 0) void page_alloc_init_cpuhp(void); -int decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp); +bool decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp); void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp); void drain_all_pages(struct zone *zone); void drain_local_pages(struct zone *zone); diff --git a/mm/page_alloc.c b/mm/page_alloc.c index d1d037f97c5f..77e7d9a5f149 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -2561,10 +2561,10 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order, * Called from the vmstat counter updater to decay the PCP high. * Return whether there are addition works to do. */ -int decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp) +bool decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp) { int high_min, to_drain, batch; - int todo = 0; + bool todo; high_min = READ_ONCE(pcp->high_min); batch = READ_ONCE(pcp->batch); @@ -2577,7 +2577,7 @@ int decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp) pcp->high = max3(pcp->count - (batch << CONFIG_PCP_BATCH_SCALE_MAX), pcp->high - (pcp->high >> 3), high_min); if (pcp->high > high_min) - todo++; + todo = true; } to_drain = pcp->count - pcp->high; @@ -2585,7 +2585,7 @@ int decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp) spin_lock(&pcp->lock); free_pcppages_bulk(zone, to_drain, pcp, 0); spin_unlock(&pcp->lock); - todo++; + todo = true; } return todo; diff --git a/mm/vmstat.c b/mm/vmstat.c index 71cd1ceba191..1f74a3517ab2 100644 --- a/mm/vmstat.c +++ b/mm/vmstat.c @@ -771,25 +771,25 @@ EXPORT_SYMBOL(dec_node_page_state); /* * Fold a differential into the global counters. - * Returns the number of counters updated. + * Returns whether counters were updated. */ static int fold_diff(int *zone_diff, int *node_diff) { int i; - int changes = 0; + bool changed = false; for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) if (zone_diff[i]) { atomic_long_add(zone_diff[i], &vm_zone_stat[i]); - changes++; + changed = true; } for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++) if (node_diff[i]) { atomic_long_add(node_diff[i], &vm_node_stat[i]); - changes++; + changed = true; } - return changes; + return changed; } /* @@ -806,16 +806,16 @@ static int fold_diff(int *zone_diff, int *node_diff) * with the global counters. These could cause remote node cache line * bouncing and will have to be only done when necessary. * - * The function returns the number of global counters updated. + * The function returns whether global counters were updated. */ -static int refresh_cpu_vm_stats(bool do_pagesets) +static bool refresh_cpu_vm_stats(bool do_pagesets) { struct pglist_data *pgdat; struct zone *zone; int i; int global_zone_diff[NR_VM_ZONE_STAT_ITEMS] = { 0, }; int global_node_diff[NR_VM_NODE_STAT_ITEMS] = { 0, }; - int changes = 0; + bool changed = false; for_each_populated_zone(zone) { struct per_cpu_zonestat __percpu *pzstats = zone->per_cpu_zonestats; @@ -839,7 +839,7 @@ static int refresh_cpu_vm_stats(bool do_pagesets) if (do_pagesets) { cond_resched(); - changes += decay_pcp_high(zone, this_cpu_ptr(pcp)); + changed |= decay_pcp_high(zone, this_cpu_ptr(pcp)); #ifdef CONFIG_NUMA /* * Deal with draining the remote pageset of this @@ -861,13 +861,13 @@ static int refresh_cpu_vm_stats(bool do_pagesets) } if (__this_cpu_dec_return(pcp->expire)) { - changes++; + changed = true; continue; } if (__this_cpu_read(pcp->count)) { drain_zone_pages(zone, this_cpu_ptr(pcp)); - changes++; + changed = true; } #endif } @@ -887,8 +887,8 @@ static int refresh_cpu_vm_stats(bool do_pagesets) } } - changes += fold_diff(global_zone_diff, global_node_diff); - return changes; + changed |= fold_diff(global_zone_diff, global_node_diff); + return changed; } /* -- 2.47.3 drain_pages_zone completely drains a zone of its pcp free pages by repeatedly calling free_pcppages_bulk until pcp->count reaches 0. In this loop, it already performs batched calls to ensure that free_pcppages_bulk isn't called to free too many pages at once, and relinquishes & reacquires the lock between each call to prevent lock starvation from other processes. However, the current batching does not prevent lock starvation. The current implementation creates batches of pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX, which has been seen in Meta workloads to be up to 64 << 5 == 2048 pages. While it is true that CONFIG_PCP_BATCH_SCALE_MAX is a config and indeed can be adjusted by the system admin to be any number from 0 to 6, it's default value of 5 is still too high to be reasonable for any system. Instead, let's create batches of pcp->batch pages, which gives a more reasonable 64 pages per call to free_pcppages_bulk. This gives other processes a chance to grab the lock and prevents starvation. Each individual call to drain_pages_zone may take longer, but we avoid the worst case scenario of completely starving out other system-critical threads from acquiring the pcp lock while 2048 pages are freed one-by-one. Signed-off-by: Joshua Hahn --- mm/page_alloc.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 77e7d9a5f149..b861b647f184 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -2623,8 +2623,7 @@ static void drain_pages_zone(unsigned int cpu, struct zone *zone) spin_lock(&pcp->lock); count = pcp->count; if (count) { - int to_drain = min(count, - pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX); + int to_drain = min(count, pcp->batch); free_pcppages_bulk(zone, to_drain, pcp, 0); count -= to_drain; -- 2.47.3 It is possible for pcp->count - pcp->high to exceed pcp->batch by a lot. When this happens, we should perform batching to ensure that free_pcppages_bulk isn't called with too many pages to free at once and starve out other threads that need the pcp lock. Since we are still only freeing the difference between the initial pcp->count and pcp->high values, there should be no change to how many pages are freed. Suggested-by: Chris Mason Suggested-by: Andrew Morton Co-developed-by: Johannes Weiner Signed-off-by: Joshua Hahn --- mm/page_alloc.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index b861b647f184..467e524a99df 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -2563,7 +2563,7 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order, */ bool decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp) { - int high_min, to_drain, batch; + int high_min, to_drain, to_drain_batched, batch; bool todo; high_min = READ_ONCE(pcp->high_min); @@ -2581,11 +2581,14 @@ bool decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp) } to_drain = pcp->count - pcp->high; - if (to_drain > 0) { + while (to_drain > 0) { + to_drain_batched = min(to_drain, batch); spin_lock(&pcp->lock); - free_pcppages_bulk(zone, to_drain, pcp, 0); + free_pcppages_bulk(zone, to_drain_batched, pcp, 0); spin_unlock(&pcp->lock); todo = true; + + to_drain -= to_drain_batched; } return todo; -- 2.47.3 Before returning, free_frozen_page_commit calls free_pcppages_bulk using nr_pcp_free to determine how many pages can appropritately be freed, based on the tunable parameters stored in pcp. While this number is an accurate representation of how many pages should be freed in total, it is not an appropriate number of pages to free at once using free_pcppages_bulk, since we have seen the value consistently go above 2000 in the Meta fleet on larger machines. As such, perform batched page freeing in free_pcppages_bulk by using pcp->batch member. In order to ensure that other processes are not starved of the pcp (and zone) lock, free the pcp lock. Note that because free_frozen_page_commit now performs a spinlock inside the function (and can fail), the function may now return with a freed pcp. To handle this, return true if the pcp is locked on exit and false otherwise. In addition, since free_frozen_page_commit must now be aware of what UP flags were stored at the time of the spin lock, and because we must be able to report new UP flags to the callers, add a new unsigned long* parameter UP_flags to keep track of this. Suggested-by: Chris Mason Co-developed-by: Johannes Weiner Signed-off-by: Joshua Hahn --- mm/page_alloc.c | 47 +++++++++++++++++++++++++++++++++++++---------- 1 file changed, 37 insertions(+), 10 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 467e524a99df..50589ff0a9c6 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -2821,11 +2821,19 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone, return high; } -static void free_frozen_page_commit(struct zone *zone, +/* + * Tune pcp alloc factor and adjust count & free_count. Free pages to bring the + * pcp's watermarks below high. + * + * May return a freed pcp, if during page freeing the pcp spinlock cannot be + * reacquired. Return true if pcp is locked, false otherwise. + */ +static bool free_frozen_page_commit(struct zone *zone, struct per_cpu_pages *pcp, struct page *page, int migratetype, - unsigned int order, fpi_t fpi_flags) + unsigned int order, fpi_t fpi_flags, unsigned long *UP_flags) { int high, batch; + int to_free, to_free_batched; int pindex; bool free_high = false; @@ -2864,17 +2872,34 @@ static void free_frozen_page_commit(struct zone *zone, * Do not attempt to take a zone lock. Let pcp->count get * over high mark temporarily. */ - return; + return true; } high = nr_pcp_high(pcp, zone, batch, free_high); - if (pcp->count >= high) { - free_pcppages_bulk(zone, nr_pcp_free(pcp, batch, high, free_high), - pcp, pindex); + to_free = nr_pcp_free(pcp, batch, high, free_high); + while (to_free > 0 && pcp->count >= high) { + to_free_batched = min(to_free, batch); + free_pcppages_bulk(zone, to_free_batched, pcp, pindex); if (test_bit(ZONE_BELOW_HIGH, &zone->flags) && zone_watermark_ok(zone, 0, high_wmark_pages(zone), ZONE_MOVABLE, 0)) clear_bit(ZONE_BELOW_HIGH, &zone->flags); + + high = nr_pcp_high(pcp, zone, batch, free_high); + to_free -= to_free_batched; + if (pcp->count >= high) { + pcp_spin_unlock(pcp); + pcp_trylock_finish(*UP_flags); + + pcp_trylock_prepare(*UP_flags); + pcp = pcp_spin_trylock(zone->per_cpu_pageset); + if (!pcp) { + pcp_trylock_finish(*UP_flags); + return false; + } + } } + + return true; } /* @@ -2922,8 +2947,9 @@ static void __free_frozen_pages(struct page *page, unsigned int order, pcp_trylock_prepare(UP_flags); pcp = pcp_spin_trylock(zone->per_cpu_pageset); if (pcp) { - free_frozen_page_commit(zone, pcp, page, migratetype, order, fpi_flags); - pcp_spin_unlock(pcp); + if (free_frozen_page_commit(zone, pcp, page, migratetype, order, + fpi_flags, &UP_flags)) + pcp_spin_unlock(pcp); } else { free_one_page(zone, page, pfn, order, fpi_flags); } @@ -3022,8 +3048,9 @@ void free_unref_folios(struct folio_batch *folios) migratetype = MIGRATE_MOVABLE; trace_mm_page_free_batched(&folio->page); - free_frozen_page_commit(zone, pcp, &folio->page, migratetype, - order, FPI_NONE); + if (!free_frozen_page_commit(zone, pcp, &folio->page, + migratetype, order, FPI_NONE, &UP_flags)) + pcp = NULL; } if (pcp) { -- 2.47.3