linux-kernel - Re: [patch 2/2] keep on ticking if oprofile is active

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Date:	Fri, 29 May 2009 15:14:38 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
cc:	linux-kernel@...r.kernel.org, Rob van der Heij <rvdheij@...il.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>, john stultz <johnstul@...ibm.com>
Subject: Re: [patch 2/2] keep on ticking if oprofile is active

On Fri, 29 May 2009, Martin Schwidefsky wrote:
> On Thu, 28 May 2009 22:29:38 +0200 (CEST)
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > On Thu, 28 May 2009, Martin Schwidefsky wrote:
> > > From: Martin Schwidefsky <schwidefsky@...ibm.com>
> > > 
> > > On a NOHZ system with oprofile enabled the timer tick should not be
> > > stopped when a cpu goes idle. Oprofile needs the pt_regs structure
> > > of the interrupt and allocates memory in the ring buffer for each
> > > sample. Current a maximum of 1 tick is accounted with oprofile if a
> > > cpu sleeps for a longer period of time. This does bad things to the
> > > percentages in the oprofile output. To postpone the oprofile tick to
> > > tick_nohz_restart_sched_tick analog to the in kernel profiler is not
> > > possible as there is no pt_regs structure in the context the
> > > tick_nohz_restart_sched_tick function is called and it is not a good
> > > idea to create hundreds of samples at once.
> > 
> >   Sigh. That's stupid.
> 
> What is stupid, the bug or the fix?

  The bug :)
  
> >   OTOH, thinking more about the patch itself it might be even useful
> >   for things aside oprofile. Runtime switching from and to nohz mode
> >   for debugging or evaluation purposes comes to my mind. That would
> >   need some sysfs interface, but that's not too hard to do.
> 
> That should be no problem. We used to have the hz_timer system control
> with the old no-tick solution on s390. 
> 
> >   So yeah, I think we should satisfy oprofile needs and utilize it further.
> 
> Ok, so you are in principle fine with the patch?

Yup.

	tglx
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