linux-kernel - Re: [PATCH V3]hrtimer: Fix a performance regression by disable reprogramming in remove

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Message-ID: <20130730093519.GP3008@twins.programming.kicks-ass.net>
Date:	Tue, 30 Jul 2013 11:35:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ethan Zhao <ethan.kernel@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, johlstei@...eaurora.org,
	Yinghai Lu <yinghai@...nel.org>, Jin Feng <joe.jin@...cle.com>
Subject: Re: [PATCH V3]hrtimer: Fix a performance regression by disable
 reprogramming in remove_hrtimer

On Tue, Jul 30, 2013 at 05:12:49PM +0800, Ethan Zhao wrote:
> On Mon, Jul 29, 2013 at 6:18 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > The test case does not involve anything hrtimer related. Do you have
> > CONFIG_SCHED_HRTICK enabled?
> >
> 
> Yes.  it is default configured in stable release.
> CONFIG_SCHED_HRTICK=y

Should still be disabled by default even if supported:

# grep HRTICK kernel/sched/features.h
SCHED_FEAT(HRTICK, false)


> > First of all we want to know, which particular hrtimer is causing that
> > issue. If it is the hrtick one, then I really have to ask why you want
> > to use it at all in such a high performance scenario.
> >
> >  Any advice about the HZ in high performance scenario ? hrtimer tick
> Is not fit for high performance ?

Hence why its disabled, programming the timer hardware is too expensive.
But since you didn't even know that I suspect you aren't in fact using
it.

It would be good if you could do what Thomas suggested and look at which
timer is actually active during your workload.
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