linux-kernel - Re: [tip:perf/core] tracing: Add unstable sched clock note to the warning

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Message-ID: <1297965529.23343.922.camel@gandalf.stny.rr.com>
Date:	Thu, 17 Feb 2011 12:58:49 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, jolsa@...hat.com,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] tracing: Add unstable sched clock note to the
 warning

On Thu, 2011-02-17 at 18:18 +0100, Ingo Molnar wrote:
> * Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > On Thu, 2011-02-17 at 15:00 +0000, tip-bot for Jiri Olsa wrote:
> > > Commit-ID:  5e38ca8f3ea423442eaafe1b7e206084aa38120a
> > > Gitweb:     http://git.kernel.org/tip/5e38ca8f3ea423442eaafe1b7e206084aa38120a
> > > Author:     Jiri Olsa <jolsa@...hat.com>
> > > AuthorDate: Wed, 2 Feb 2011 13:28:18 +0100
> > > Committer:  Steven Rostedt <rostedt@...dmis.org>
> > > CommitDate: Tue, 8 Feb 2011 11:57:22 -0500
> > > 
> > > tracing: Add unstable sched clock note to the warning
> > > 
> > > The warning "Delta way too big" warning might appear on a system with
> > > unstable shed clock right after the system is resumed and tracing
> > > was enabled during the suspend.
> > > 
> > > Since it's not realy bug, and the unstable sched clock is working
> > > fast and reliable otherwise, Steven suggested to keep using the
> > > sched clock in any case and just to make note in the warning itself.
> > > 
> > 
> > 
> > Peter Zijlstra just informed me on IRC that the commit:
> > 
> > cd7240c0b900eb6d690ccee088a6c9b46dae815a
> > x86, tsc, sched: Recompute cyc2ns_offset's during resume from sleep
> > states
> > 
> > Prevents the clock from going backwards triggering the warn on. If this
> > is the case, I think we can revert this patch.
> > 
> > I'll wait on the original reporters of the bug to find out. If the clock
> > does not go backwards when the above commit is applied, then this patch
> > is not needed.
> > 
> > If this is the case, do you want me to rebase without it, or should we
> > just revert it?
> 
> A revert would be nicer (with an explanation in it), as i've already pushed out the 
> perc/core tree.
> 
> If you do the revert today against the current perf/core tip (bee96907383e) then i 
> can pull it in a fast-forward way and we'd minimalize the window.

I've just been informed that even with this patch, the clock still goes
backwards after resume.

-- Steve


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