linux-kernel - Re: Bisect results for 4.4.1-rt[4,5]

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Message-ID: <56CF0A63.7010203@linutronix.de>
Date:	Thu, 25 Feb 2016 15:06:27 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Tim Sander <tim@...eglstein.org>
Cc:	linux-rt-users <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>, rostedt@...dmis.org,
	John Kacur <jkacur@...hat.com>
Subject: Re: Bisect results for 4.4.1-rt[4,5]

On 02/17/2016 09:14 AM, Tim Sander wrote:
> Hi Sebastian

Hi Tim,

> I have done a bisect run, its a rather innocent looking on liner which seems 
> to cause the problems. The numbers where reasonably stable so i am pretty 
> confident that this is the patch giving ~26µs additional latency on the Altera
> SOC plattform:
> 
> eec2bf477ac674583a7d73b9d00f47c528b7266d is the first bad commit
> commit eec2bf477ac674583a7d73b9d00f47c528b7266d
> Author: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Date:   Thu Feb 4 16:38:10 2016 +0100
> 
>     kernel/perf: mark perf_cpu_context's timer as irqsafe
>     
>     Otherwise we get a WARN_ON() backtrace and some events are reported as
>     "not counted".
>     
>     Cc: stable-rt@...r.kernel.org
>     Reported-by: Yang Shi <yang.shi@...aro.org>
>     Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Seriously? That patch? I played a little and I doubt seriously that
this patch has something to do with it.
So before that patch you would have a warn_on spotted and complained if
that timer would fire. So that is one reason why I doubt that this
patch is in charge of the 25us.

If I add a printk() to that timer I don't see it under "normal"
circumstances. However I do
 perf_4.3 stat -e
branches,branch-misses,bus-cycles,cache-misses,cache-references,cycles,instructions
apt-get update

then I see them fire.

> Best Regards
> Tim

Sebastian