The patch titled Subject: mm, meminit: recalculate pcpu batch and high limits after init completes has been added to the -mm tree. Its filename is mm-meminit-recalculate-pcpu-batch-and-high-limits-after-init-completes.patch

This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/mm-meminit-recalculate-pcpu-batch-a... and later at http://ozlabs.org/~akpm/mmotm/broken-out/mm-meminit-recalculate-pcpu-batch-a...

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------------------------------------------------------ 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

Link: 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 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 | 8 ++++++++ 1 file changed, 8 insertions(+)

--- 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. */ _

Patches currently in -mm which might be from mgorman@techsingularity.net are

mm-meminit-recalculate-pcpu-batch-and-high-limits-after-init-completes.patch mm-pcp-share-common-code-between-memory-hotplug-and-percpu-sysctl-handler.patch mm-pcpu-make-zone-pcp-updates-and-reset-internal-to-the-mm.patch