linux-kernel - [rcu] 026ad2835: +30% increased context switches

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Date:	Sat, 4 Jan 2014 00:47:39 +0800
From:	fengguang.wu@...el.com
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, lkp@...ux.intel.com
Subject: [rcu] 026ad2835: +30% increased context switches

Hi Paul,

We are running 0day performance tests for both new and old kernels,
and happen to notice that your old commit 026ad2835 ("rcu: Drive
quiescent-state-forcing delay from HZ") increased the context switches
(accidentally?) in the will-it-scale/pread1 test case. Just would like
to confirm with you if this is expected. Attached is the kconfig. The
test machine is a 4S ivybridge system that has 120 CPU threads.

Comparing first bad commit with its parent:

9a5739d73f9369b  026ad2835ce6202069e7aa0b1
---------------  -------------------------
       782 ~ 0%     +30.1%       1017 ~ 0%  vmstat.system.cs
     13622 ~ 0%     +26.2%      17195 ~ 0%  time.involuntary_context_switches

Comparing Linux releases:

          v3.10                      v3.11                      v3.12                  v3.13-rc6
---------------  -------------------------  -------------------------  -------------------------
       788 ~ 0%     +29.9%       1023 ~ 3%     +34.9%       1063 ~ 1%     +38.6%       1091 ~ 0%  vmstat.system.cs
     13807 ~ 1%     -18.1%      11306 ~40%     +24.1%      17128 ~ 0%     +25.9%      17376 ~ 0%  time.involuntary_context_switches

samples from the bisect good/bad commits

                          time.involuntary_context_switches

   18000 ++-----------------------------------------------------------------+
         |                 O   O                                            |
   17500 O+  OO                   O   O O  O   O                            |
   17000 ++O    O O O OO O   O   O  O     O  O                              |
         |                                                                  |
   16500 ++                                                                 |
   16000 ++                                                                 |
         |                                                                  |
   15500 ++                                                                 |
   15000 ++                                                                 |
         |                                                                  |
   14500 ++                                                                 |
   14000 *+                                                                 |
         | *.**.   .*.* .*.*.*. .* .*.*.*. *.*. .*.**.*. .*.*.**.*.   .*    |
   13500 ++-----*-*----*-------*--*-------*----*--------*----------*-*--*-*-*


                                  vmstat.system.cs

   1050 ++------------------------------------------------------------------+
        O O O OO O O O O OO O O   O  O   O O   O                            |
   1000 ++                      O   O  O     O                              |
        |                                                                   |
        |                                                                   |
    950 ++                                                                  |
        |                                                                   |
    900 ++                                                                  |
        |                                                                   |
    850 ++                                                                  |
        |                                                                   |
        |                    .*.                           *.               |
    800 *+*. .**.*.*.*.*.**.*   *.*.**.*. .*.   *.*.*.*.*. : *.*.   .* .*.  |
        |   *                            *   *.*          *      *.*  *   *.*
    750 ++------------------------------------------------------------------+

Thanks,
Fengguang

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