linux-kernel - Re: [BUG, bisect] hrtimer: severe lag after suspend & resume

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Date:	Fri, 5 Jun 2015 11:14:10 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	John Stultz <john.stultz@...aro.org>
cc:	Jeremiah Mahler <jmmahler@...il.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [BUG, bisect] hrtimer: severe lag after suspend & resume

On Thu, 4 Jun 2015, John Stultz wrote:
> On Wed, Jun 3, 2015 at 5:56 PM, Jeremiah Mahler <jmmahler@...il.com> wrote:
> So I suspect the problem is the change to clock_was_set_seq in
> timekeeping_update is done prior to mirroring the time state to the
> shadow-timekeeper. Thus the next time we do update_wall_time() the
> updated sequence is overwritten by whats in the shadow copy. The
> attached patch moving the modification up seems to avoid the issue for
> me.

Duh, yes.
 
> Thomas:  Looking at the problematic change, I'm not a big fan of it.
> Caching timekeeping state here in the hrtimer code has been a source
> of bugs in the past, and I'm not sure I see how avoiding copying
> 24bytes is that big of a win. Especially since it adds more state to
> the timekeeper and hrtimer base that we have to read and mange.

It's not about copying 24 bytes. It's about touching 3 cache lines for
nothing. In situations where we run high frequency periodic timers on
clock monotonic and nothing is going on in the other clock domains,
which is a pretty common situation, this is measurable in terms of
cache utilization. I went great length to optimize the cache footprint
and access patterns and that unconditional update really makes a
measurable difference.

Thanks,

	tglx
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