linux-kernel - Re: [PATCH v4 2/3] mm/free_pcppages_bulk: do not hold lock when picking pages to free

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Date:   Thu, 1 Mar 2018 16:01:05 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Aaron Lu <aaron.lu@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Huang Ying <ying.huang@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Kemi Wang <kemi.wang@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v4 2/3] mm/free_pcppages_bulk: do not hold lock when
 picking pages to free

On Thu,  1 Mar 2018 14:28:44 +0800 Aaron Lu <aaron.lu@...el.com> 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.

But it's a loss for uniprocessor systems: it adds more code and adds an
additional pass across a list.