linux-kernel - Re: [PATCH 1/2] mm: swap: make page

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Message-ID: <94283259-a0ee-d134-48da-cf5400baaba1@linux.alibaba.com>
Date:   Fri, 13 Mar 2020 12:54:04 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: swap: make page_evictable() inline



On 3/13/20 12:50 PM, Shakeel Butt wrote:
> On Fri, Mar 13, 2020 at 12:46 PM Yang Shi <yang.shi@...ux.alibaba.com> wrote:
>>
>>
>> On 3/13/20 12:33 PM, Shakeel Butt wrote:
>>> On Fri, Mar 13, 2020 at 11:34 AM Yang Shi <yang.shi@...ux.alibaba.com> wrote:
>>>> When backporting commit 9c4e6b1a7027 ("mm, mlock, vmscan: no more
>>>> skipping pagevecs") to our 4.9 kernel, our test bench noticed around 10%
>>>> down with a couple of vm-scalability's test cases (lru-file-readonce,
>>>> lru-file-readtwice and lru-file-mmap-read).  I didn't see that much down
>>>> on my VM (32c-64g-2nodes).  It might be caused by the test configuration,
>>>> which is 32c-256g with NUMA disabled and the tests were run in root memcg,
>>>> so the tests actually stress only one inactive and active lru.  It
>>>> sounds not very usual in mordern production environment.
>>>>
>>>> That commit did two major changes:
>>>> 1. Call page_evictable()
>>>> 2. Use smp_mb to force the PG_lru set visible
>>>>
>>>> It looks they contribute the most overhead.  The page_evictable() is a
>>>> function which does function prologue and epilogue, and that was used by
>>>> page reclaim path only.  However, lru add is a very hot path, so it
>>>> sounds better to make it inline.  However, it calls page_mapping() which
>>>> is not inlined either, but the disassemble shows it doesn't do push and
>>>> pop operations and it sounds not very straightforward to inline it.
>>>>
>>>> Other than this, it sounds smp_mb() is not necessary for x86 since
>>>> SetPageLRU is atomic which enforces memory barrier already, replace it
>>>> with smp_mb__after_atomic() in the following patch.
>>>>
>>>> With the two fixes applied, the tests can get back around 5% on that
>>>> test bench and get back normal on my VM.  Since the test bench
>>>> configuration is not that usual and I also saw around 6% up on the
>>>> latest upstream, so it sounds good enough IMHO.
>>>>
>>>> The below is test data (lru-file-readtwice throughput) against the v5.6-rc4:
>>>>           mainline        w/ inline fix
>>>>             150MB            154MB
>>>>
>>> What is the test setup for the above experiment? I would like to get a repro.
>> Just startup a VM with two nodes, then run case-lru-file-readtwice or
>> case-lru-file-readonce in vm-scalability in root memcg or with memcg
>> disabled.  Then get the average throughput (dd result) from the test.
>> Our test bench uses the script from lkp, but I just ran it manually.
>> Single node VM should be more obvious showed in my test.
>>
> Thanks, I will try this on a real machine

Real machine should be better. Our test bench is bare metal with NUMA 
disabled. On my test VM it is not that obvious.