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SubjectRe: [PATCH v4 2/3] mm/free_pcppages_bulk: do not hold lock when picking pages to free
On Thu, Mar 01, 2018 at 02:55:18PM +0100, Michal Hocko wrote:
> On Thu 01-03-18 14:28:44, Aaron Lu wrote:
> > When freeing a batch of pages from Per-CPU-Pages(PCP) back to buddy,
> > the zone->lock is held and then pages are chosen from PCP's migratetype
> > list. While there is actually no need to do this 'choose part' under
> > lock since it's PCP pages, the only CPU that can touch them is us and
> > irq is also disabled.
> >
> > Moving this part outside could reduce lock held time and improve
> > performance. Test with will-it-scale/page_fault1 full load:
> >
> > kernel Broadwell(2S) Skylake(2S) Broadwell(4S) Skylake(4S)
> > v4.16-rc2+ 9034215 7971818 13667135 15677465
> > this patch 9536374 +5.6% 8314710 +4.3% 14070408 +3.0% 16675866 +6.4%
> >
> > What the test does is: starts $nr_cpu processes and each will repeatedly
> > do the following for 5 minutes:
> > 1 mmap 128M anonymouse space;
> > 2 write access to that space;
> > 3 munmap.
> > The score is the aggregated iteration.
>
> Iteration count I assume.

Correct.

> I am still quite surprised that this would have such a large impact.

Most likely due to the cachelines for these page structures are warmed
up outside of zone->lock.

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