linux-kernel - Re: [PATCH 1/2] mm: support fastpath if NUMA is enabled with numa off

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Date:   Thu, 17 Jun 2021 13:40:52 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Janghyuck Kim <janghyuck.kim@...sung.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Atish Patra <atish.patra@....com>,
        Gavin Shan <gshan@...hat.com>,
        Zhengyuan Liu <liuzhengyuan@...kylinos.cn>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 1/2] mm: support fastpath if NUMA is enabled with numa off

On Thu, Jun 17, 2021 at 08:55:44PM +0900, Janghyuck Kim wrote:
> On Wed, Jun 16, 2021 at 06:32:50PM +0100, Matthew Wilcox wrote:
> > On Wed, Jun 16, 2021 at 05:37:41PM +0900, Janghyuck Kim wrote:
> > > Architecture might support fake node when CONFIG_NUMA is enabled but any
> > > node settings were supported by ACPI or device tree. In this case,
> > > getting memory policy during memory allocation path is meaningless.
> > > 
> > > Moreover, performance degradation was observed in the minor page fault
> > > test, which is provided by (https://protect2.fireeye.com/v1/url?k=c81407ae-978f3ea4-c8158ce1-0cc47a31384a-10187d5ead74c318&q=1&e=cbc91c9b-80e1-4ca0-b51a-9f79fad5b0c1&u=https%3A%2F%2Flkml.org%2Flkml%2F2006%2F8%2F29%2F294).
> > > Average faults/sec of enabling NUMA with fake node was 5~6 % worse than
> > > disabling NUMA. To reduce this performance regression, fastpath is
> > > introduced. fastpath can skip the memory policy checking if NUMA is
> > > enabled but it uses fake node. If architecture doesn't support fake
> > > node, fastpath affects nothing for memory allocation path.
> > 
> > This patch doesn't even apply to the current kernel, but putting that
> > aside, what's the expensive part of the current code?  That is,
> > comparing performance stats between this numa_off enabled and numa_off
> > disabled, where do you see taking a lot of time?
> > 
> 
> mempolicy related code that I skipped by this patch took a short time,
> taking only a few tens of nanoseconds that difficult to measure by
> sched_clock's degree of precision. But it can be affect the minor page
> fault test with large buffer size, because one page fault handling takes
> several ms. As I replied in previous mail, performance regression has
> been reduced from 5~6% to 2~3%.

I'm not proposing you use sched_clock.  Try perf.