linux-kernel - Re: [v2 PATCH 1/2] mm: swap: make page

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Date:   Tue, 17 Mar 2020 09:59:05 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Yang Shi <yang.shi@...ux.alibaba.com>, shakeelb@...gle.com,
        willy@...radead.org, akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 1/2] mm: swap: make page_evictable() inline

On 3/16/20 11:24 PM, Yang Shi 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@...gle.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Matthew Wilcox <willy@...radead.org>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

Thanks.