linux-kernel - Re: [v4 PATCH 1/2] mm: swap: make page

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Date:   Thu, 19 Mar 2020 10:01:54 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     shakeelb@...gle.com, vbabka@...e.cz, willy@...radead.org,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v4 PATCH 1/2] mm: swap: make page_evictable() inline

On Wed, Mar 18, 2020 at 11:02:20AM +0800, 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.
> 
> Shakeel Butt did the below test:
> 
> On a real machine with limiting the 'dd' on a single node and reading 100 GiB
> sparse file (less than a single node).  Just ran a single instance to not
> cause the lru lock contention. The cmdline used is
> "dd if=file-100GiB of=/dev/null bs=4k".  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
> 
> Fixes: 9c4e6b1a7027 ("mm, mlock, vmscan: no more skipping pagevecs")
> Cc: Matthew Wilcox <willy@...radead.org>
> Reviewed-and-Tested-by: Shakeel Butt <shakeelb@...gle.com>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>

Acked-by: Johannes Weiner <hannes@...xchg.org>