linux-kernel - Re: [regression] cross core scheduling frequency drop bisected to 0c313cb20732

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Date:	Fri, 8 Apr 2016 08:45:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [regression] cross core scheduling frequency drop bisected to
 0c313cb20732

On Fri, Apr 08, 2016 at 07:20:54AM +0200, Mike Galbraith wrote:
> Greetings,
> 
> While measuring current NO_HZ cost to light tasks jabbering cross core
> at high frequency (~7% max), I noticed that master lost an improvement
> for same acquired in 4.5, so bisected it.
> 
> 4.5.0
> homer:~ # taskset 0xc pipe-test 1
> 2.367681 usecs/loop -- avg 2.367681 844.7 KHz
> 2.372502 usecs/loop -- avg 2.368163 844.5 KHz
> 2.342506 usecs/loop -- avg 2.365597 845.5 KHz
> 2.383029 usecs/loop -- avg 2.367341 844.8 KHz
> 2.321859 usecs/loop -- avg 2.362792 846.5 KHz   1.00
> 
> master
> homer:~ # taskset 0xc pipe-test 1
> 2.797656 usecs/loop -- avg 2.797656 714.9 KHz
> 2.804518 usecs/loop -- avg 2.798342 714.7 KHz
> 2.804206 usecs/loop -- avg 2.798929 714.6 KHz
> 2.802887 usecs/loop -- avg 2.799324 714.5 KHz
> 2.801577 usecs/loop -- avg 2.799550 714.4 KHz   0.84
> 
> master 0c313cb20732 reverted
> homer:~ # !taskset
> homer:~ # taskset 0xc pipe-test 1
> 2.277494 usecs/loop -- avg 2.277494 878.2 KHz
> 2.320979 usecs/loop -- avg 2.281843 876.5 KHz
> 2.272750 usecs/loop -- avg 2.280933 876.8 KHz
> 2.272209 usecs/loop -- avg 2.280061 877.2 KHz
> 2.277279 usecs/loop -- avg 2.279783 877.3 KHz   1.03
> 
> 0c313cb207326f759a58f486214288411b25d4cf is the first bad commit
> commit 0c313cb207326f759a58f486214288411b25d4cf
> Author: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Date:   Sun Mar 20 01:33:35 2016 +0100
> 
>     cpuidle: menu: Fall back to polling if next timer event is near
>     

Cute, I thought you used governor=performance for your runs?