From: Mel Gorman mgorman@techsingularity.net Subject: mm, meminit: recalculate pcpu batch and high limits after init completes

Deferred memory initialisation updates zone->managed_pages during the initialisation phase but before that finishes, the per-cpu page allocator (pcpu) calculates the number of pages allocated/freed in batches as well as the maximum number of pages allowed on a per-cpu list. As zone->managed_pages is not up to date yet, the pcpu initialisation calculates inappropriately low batch and high values.

This increases zone lock contention quite severely in some cases with the degree of severity depending on how many CPUs share a local zone and the size of the zone. A private report indicated that kernel build times were excessive with extremely high system CPU usage. A perf profile indicated that a large chunk of time was lost on zone->lock contention.

This patch recalculates the pcpu batch and high values after deferred initialisation completes for every populated zone in the system. It was tested on a 2-socket AMD EPYC 2 machine using a kernel compilation workload -- allmodconfig and all available CPUs.

mmtests configuration: config-workload-kernbench-max Configuration was modified to build on a fresh XFS partition.

kernbench 5.4.0-rc3 5.4.0-rc3 vanilla resetpcpu-v2 Amean user-256 13249.50 ( 0.00%) 16401.31 * -23.79%* Amean syst-256 14760.30 ( 0.00%) 4448.39 * 69.86%* Amean elsp-256 162.42 ( 0.00%) 119.13 * 26.65%* Stddev user-256 42.97 ( 0.00%) 19.15 ( 55.43%) Stddev syst-256 336.87 ( 0.00%) 6.71 ( 98.01%) Stddev elsp-256 2.46 ( 0.00%) 0.39 ( 84.03%)

5.4.0-rc3 5.4.0-rc3 vanilla resetpcpu-v2 Duration User 39766.24 49221.79 Duration System 44298.10 13361.67 Duration Elapsed 519.11 388.87

The patch reduces system CPU usage by 69.86% and total build time by 26.65%. The variance of system CPU usage is also much reduced.

Before, this was the breakdown of batch and high values over all zones was.

256 batch: 1 256 batch: 63 512 batch: 7 256 high: 0 256 high: 378 512 high: 42

512 pcpu pagesets had a batch limit of 7 and a high limit of 42. After the patch:

256 batch: 1 768 batch: 63 256 high: 0 768 high: 378

[mgorman@techsingularity.net: fix merge/linkage snafu] Link: http://lkml.kernel.org/r/20191023084705.GD3016@techsingularity.netLink: http://lkml.kernel.org/r/20191021094808.28824-2-mgorman@techsingularity.net Signed-off-by: Mel Gorman mgorman@techsingularity.net Acked-by: Michal Hocko mhocko@suse.com Acked-by: Vlastimil Babka vbabka@suse.cz Acked-by: David Hildenbrand david@redhat.com Cc: Matt Fleming matt@codeblueprint.co.uk Cc: Thomas Gleixner tglx@linutronix.de Cc: Borislav Petkov bp@alien8.de Cc: Qian Cai cai@lca.pw Cc: stable@vger.kernel.org [4.1+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/page_alloc.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/mm/page_alloc.c~mm-meminit-recalculate-pcpu-batch-and-high-limits-after-init-completes +++ a/mm/page_alloc.c @@ -1948,6 +1948,14 @@ void __init page_alloc_init_late(void) wait_for_completion(&pgdat_init_all_done_comp);

/* + * The number of managed pages has changed due to the initialisation + * so the pcpu batch and high limits needs to be updated or the limits + * will be artificially small. + */ + for_each_populated_zone(zone) + zone_pcp_update(zone); + + /* * We initialized the rest of the deferred pages. Permanently disable * on-demand struct page initialization. */ @@ -8514,7 +8522,6 @@ void free_contig_range(unsigned long pfn WARN(count != 0, "%d pages are still in use!\n", count); }

-#ifdef CONFIG_MEMORY_HOTPLUG /* * The zone indicated has a new number of managed_pages; batch sizes and percpu * page high values need to be recalulated. @@ -8528,7 +8535,6 @@ void __meminit zone_pcp_update(struct zo per_cpu_ptr(zone->pageset, cpu)); mutex_unlock(&pcp_batch_high_lock); } -#endif

void zone_pcp_reset(struct zone *zone) { _