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SubjectRe: [PATCH V3]hrtimer: Fix a performance regression by disable reprogramming in remove_hrtimer
Peter and tglx,
Some other tough hacking and testing with result FYI,
With the default kernel 2.6.32-279.19.1.el6.x86_64 in CentOS 6.3 running on my ASUS 4 core Intel i5 server, almost got the best performance of

[root@localhost ~]# time ./pipe-test-1m

real 0m7.704s
user 0m0.047s
sys 0m4.815s
[root@localhost ~]# time ./pipe-test-1m

real 0m8.000s
user 0m0.071s
sys 0m5.035s
[root@localhost ~]# time ./pipe-test-1m

real 0m7.386s
user 0m0.086s
sys 0m4.591s
[root@localhost ~]# time ./pipe-test-1m

real 0m7.919s
user 0m0.064s
sys 0m4.912s
[root@localhost ~]# time ./pipe-test-1m

real 0m7.949s
user 0m0.083s
sys 0m4.917s

[root@localhost ~]# time ./pipe-test-1m
real 0m7.913s
user 0m0.070s
sys 0m4.903s
[root@localhost ~]# time ./pipe-test-1m

real 0m7.953s
user 0m0.092s
sys 0m4.881s
[root@localhost ~]# time ./pipe-test-1m

real 0m8.059s
user 0m0.108s
sys 0m5.037s
[root@localhost ~]#

Then compiled and boot stable 3.11.0-rc3 with default configuration, redid the same test. got very bad performance:
root@localhost ~]# uname -a
Linux localhost 3.11.0-rc3 #4 SMP Wed Jul 31 16:10:56 EDT 2013 x86_64 x86_64 x86_64 GNU/Linux

real 0m10.730s
user 0m0.245s
sys 0m6.596s
[root@localhost ~]# time ./pipe-test-1m

real 0m10.661s
user 0m0.218s
sys 0m6.520s
[root@localhost ~]# time ./pipe-test-1m

real 0m10.699s
user 0m0.233s
sys 0m6.534s
[root@localhost ~]# time ./pipe-test-1m

real 0m10.616s
user 0m0.191s
sys 0m6.505s
[root@localhost ~]# time ./pipe-test-1m

real 0m10.546s
user 0m0.214s
sys 0m6.441s

[root@localhost ~]# time ./pipe-test-1m

real 0m10.631s
user 0m0.204s
sys 0m6.509s

First 'tough' hacking is disable the reprogramming in _remove_hrtimer() within 3.11-rc3 code and redo the test.
much better.

root@localhost ~]# time ./pipe-test-1m

real 0m9.447s
user 0m0.227s
sys 0m5.900s
[root@localhost ~]# time ./pipe-test-1m

real 0m9.507s
user 0m0.226s
sys 0m5.922s
[root@localhost ~]# time ./pipe-test-1m

real 0m9.495s
user 0m0.228s
sys 0m5.916s
[root@localhost ~]# time ./pipe-test-1m

real 0m9.470s
user 0m0.229s
sys 0m5.938s
[root@localhost ~]# time ./pipe-test-1m

real 0m9.484s
user 0m0.269s
sys 0m5.875s
[root@localhost ~]# time ./pipe-test-1m

real 0m9.328s
user 0m0.242s
sys 0m5.767s

While I monitor the wake-up with powertop, got
Top causes for wakeups:
98.5% ( inf) <kernel IPI> : Rescheduling interrupts
0.5% ( inf) swapper/3 : hrtimer_start_range_ns (tick_sched_timer)
0.3% ( inf) swapper/2 : hrtimer_start_range_ns (tick_sched_timer)
0.2% ( inf) swapper/1 : hrtimer_start_range_ns (tick_sched_timer)
0.2% ( inf) swapper/0 : hrtimer_start_range_ns (tick_sched_timer)

So I did the second tough hacking, commented out the rescheduling IPI sending in following function and re-did the test.

diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index 4137890..c27f04f 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -137,7 +137,7 @@ static inline void play_dead(void)

static inline void smp_send_reschedule(int cpu)
- smp_ops.smp_send_reschedule(cpu);
+ /* smp_ops.smp_send_reschedule(cpu); */

Got the performance as best as 2.6.32 kernel and the scheduling seems also OK.

root@localhost ~]# time ./pipe-test-1m
real 0m7.661s
user 0m0.179s
sys 0m4.880s
[root@localhost ~]# time ./pipe-test-1m

real 0m7.473s
user 0m0.189s
sys 0m4.782s
[root@localhost ~]# time ./pipe-test-1m

real 0m7.658s
user 0m0.195s
sys 0m4.899s
[root@localhost ~]# time ./pipe-test-1m

real 0m7.644s
user 0m0.194s
sys 0m4.941s
[root@localhost ~]# time ./pipe-test-1m

real 0m7.694s
user 0m0.189s
sys 0m4.925s
[root@localhost ~]# time ./pipe-test-1m

real 0m7.694s
user 0m0.197s
sys 0m4.915s
[root@localhost ~]# time ./pipe-test-1m

real 0m7.597s
user 0m0.190s
sys 0m4.886s

The the two processes of pipe-test-1m and its child seem could be balanced from cpu0 to cpu3 well,
f J
14888 root 20 0 68 0 R 73.2 0.0 0:03.22 2 pip1m
14887 root 20 0 284 224 S 63.4 0.0 0:03.23 0 pip1m

And so the above tough hacking and test basicly show the No.1 expensive thing is the rescheduling IPI, and
the No.2 expensive thing is the extra hrtimer reprogramming/tick in Linux 3.11-rc3 code.
We need manage to do as less as possible rescheduling IPI and reprogramming to get better performance.
Does it(the tough hacking and the test) make sense ? and the result rational ?


在 2013-7-30,下午7:59,Peter Zijlstra <> 写道:

> On Tue, Jul 30, 2013 at 07:44:03PM +0800, Ethan Zhao wrote:
>> Got it.
>> what tglx and you mean
>> So the expensive thing maybe not inside the schedule(), but could
>> outside the scheduler(), the more bigger forever loop.
>> This is one part of what I am facing.
> Right, so it would be good if you could further diagnose the problem so
> we can come up with a solution that cures the problem while retaining
> the current 'desired' properties.
> The patch you pinpointed caused a regression in that it would wake from
> NOHZ mode far too often. Could it be that the now longer idle sections
> cause your CPU to go into deeper idle modes and you're suffering from
> idle-exit latencies?

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