On 10/21/19 11:48 AM, Mel Gorman wrote: > 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 > > Cc: stable@xxxxxxxxxxxxxxx # v4.1+ > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Acked-by: Vlastimil Babka <vbabka@xxxxxxx> > --- > mm/page_alloc.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index c0b2e0306720..f972076d0f6b 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -1947,6 +1947,14 @@ void __init page_alloc_init_late(void) > /* Block until all are initialised */ > 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. >
