Re: [v2 PATCH 1/2] mm: swap: make page_evictable() inline

[Yang Shi <yang.shi@linux.alibaba.com>]

On 3/16/20 4:46 PM, Shakeel Butt wrote:
> On Mon, Mar 16, 2020 at 3:24 PM Yang Shi <yang.shi@linux.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
>>
>> With this patch the throughput gets 2.67% up.  The data with using
>> smp_mb__after_atomic() is showed in the following patch.
>>
>> Fixes: 9c4e6b1a7027 ("mm, mlock, vmscan: no more skipping pagevecs")
>> Cc: Shakeel Butt <shakeelb@google.com>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>
> So, I tested on a real machine with limiting the 'dd' on a single node
> and reading 100 GiB sparse file (less than a single node). I just ran
> a single instance to not cause the lru lock contention. The cmd I used
> is "dd if=file-100GiB of=/dev/null bs=4k". I ran the cmd 10 times with
> drop_caches in between and measured the time it took.
>
> Without patch: 56.64143 +- 0.672 sec
>
> With patches: 56.10 +- 0.21 sec
>
> Reviewed-and-Tested-by: Shakeel Butt <shakeelb@google.com>

Thanks Shakeel. It'd better to add your test result in the commit log too.