linux-kernel - Re: [numa shrinker] 9b17c62382: -36.6% regression on sparse file copy

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Date:	Wed, 15 Jan 2014 11:18:27 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	Glauber Costa <glommer@...allels.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	lkp@...ux.intel.com
Subject: Re: [numa shrinker] 9b17c62382: -36.6% regression on sparse file copy

On Thu, Jan 09, 2014 at 10:57:15AM +0800, Fengguang Wu wrote:
> Hi Dave,
> 
> As you suggested, I added tests for ext4 and btrfs, the results are
> the same.
> 
> Then I tried running perf record for 10 seconds starting from 200s.
> (The test runs for 410s). I see several warning messages and hope
> they do not impact the accuracy too much:
> 
> [  252.608069] perf samples too long (2532 > 2500), lowering kernel.perf_event_max_sample_rate to 50000
> [  252.608863] perf samples too long (2507 > 2500), lowering kernel.perf_event_max_sample_rate to 25000
> [  252.609422] INFO: NMI handler (perf_event_nmi_handler) took too long to run: 1.389 msecs
> 
> Anyway the noticeable perf change are:
> 
> 1d3d4437eae1bb2  9b17c62382dd2e7507984b989  
> ---------------  -------------------------  
>      12.15 ~10%    +209.8%      37.63 ~ 2%  brickland2/debug2/vm-scalability/300s-btrfs-lru-file-readtwice
>      12.88 ~16%    +189.4%      37.27 ~ 0%  brickland2/debug2/vm-scalability/300s-ext4-lru-file-readtwice
>      15.24 ~ 9%    +146.0%      37.50 ~ 1%  brickland2/debug2/vm-scalability/300s-xfs-lru-file-readtwice
>      40.27         +179.1%     112.40       TOTAL perf-profile.cpu-cycles._raw_spin_lock.grab_super_passive.super_cache_count.shrink_slab.do_try_to_free_pages
> 
> 1d3d4437eae1bb2  9b17c62382dd2e7507984b989  
> ---------------  -------------------------  
>      11.91 ~12%    +218.2%      37.89 ~ 2%  brickland2/debug2/vm-scalability/300s-btrfs-lru-file-readtwice
>      12.47 ~16%    +200.3%      37.44 ~ 0%  brickland2/debug2/vm-scalability/300s-ext4-lru-file-readtwice
>      15.36 ~11%    +145.4%      37.68 ~ 1%  brickland2/debug2/vm-scalability/300s-xfs-lru-file-readtwice
>      39.73         +184.5%     113.01       TOTAL perf-profile.cpu-cycles._raw_spin_lock.put_super.drop_super.super_cache_count.shrink_slab
> 
> perf report for 9b17c62382dd2e7507984b989:
> 
> # Overhead          Command       Shared Object                                          Symbol
> # ........  ...............  ..................  ..............................................
> #
>     77.74%               dd  [kernel.kallsyms]   [k] _raw_spin_lock                            
>                          |
>                          --- _raw_spin_lock
>                             |          
>                             |--47.65%-- grab_super_passive

Oh, it's superblock lock contention, probably caused by an increase
in shrinker calls (i.e. per-node rather than global). I think we've
seen this before - can you try the two patches from Tim Chen here:

https://lkml.org/lkml/2013/9/6/353
https://lkml.org/lkml/2013/9/6/356

If they fix the problem, I'll get them into 3.14 and pushed back to
the relevant stable kernels.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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