linux-kernel - Re: [patch 1/3] sched: init rt_avg stat whenever rq comes online

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Date:	Thu, 19 Aug 2010 10:53:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"chris@...stnet.net" <chris@...stnet.net>,
	"debian00@...ceadsl.fr" <debian00@...ceadsl.fr>,
	"hpa@...or.com" <hpa@...or.com>,
	"jonathan.protzenko@...il.com" <jonathan.protzenko@...il.com>,
	"mans@...sr.com" <mans@...sr.com>,
	"psastudio@...l.ru" <psastudio@...l.ru>,
	"rjw@...k.pl" <rjw@...k.pl>,
	"stephan.eicher@....de" <stephan.eicher@....de>,
	"sxxe@....de" <sxxe@....de>,
	"thomas@...hlinux.org" <thomas@...hlinux.org>,
	"venki@...gle.com" <venki@...gle.com>,
	"wonghow@...il.com" <wonghow@...il.com>,
	"stable@...nel.org" <stable@...nel.org>, tglx <tglx@...utronix.de>
Subject: Re: [patch 1/3] sched: init rt_avg stat whenever rq comes online

On Wed, 2010-08-18 at 17:20 -0700, Suresh Siddha wrote:
> On Tue, 2010-08-17 at 01:51 -0700, Peter Zijlstra wrote:
> > On Mon, 2010-08-16 at 21:25 +0200, Peter Zijlstra wrote:
> > > You can use something like:
> > > 
> > > suspend:
> > >  __get_cpu_var(cyc2ns_suspend) = sched_clock();
> > > 
> > > resume:
> > >  for_each_possible_cpu(i)
> > >    per_cpu(cyc2ns_offset, i) += per_cpu(cyc2ns_suspend);
> > > 
> > > or something like that to keep sched_clock() stable, which is exactly
> > > what most (all?) its users expect when we report the TSC is usable. 
> > 
> > That's actually broken, you only want a single offset, otherwise we
> > de-sync the TSC, which is bad.
> > 
> > So simply store the sched_clock() value at suspend time on the single
> > CPU that is still running, then on resume make sure sched_clock()
> > continues there by adding that stamp to all CPU offsets.
> 
> 
> Peter, That might not be enough. I should add that in my Lenovo T410
> (having 2 core wsm cpu), TSC's are somehow set to a strange big value
> (for example 0xfffffffebc22f02e) after resume from S3. It looks like
> bios might be writing TSC during resume. I am not sure if this is the
> case for other OEM laptops aswell. I am checking.

ARGH, please kill all SMM support for future CPUs ;-)

Are the TSCs still sync'ed though? If so, we can still compute a offset
and continue with things, albeit it requires something like:

  local_irq_save(flags);
  __get_cpu_var(cyc2ns_offset) = 0;
  offset = cyc2ns_suspend - sched_clock();
  local_irq_restore(flags);

  for_each_possible_cpu(i)
    per_cpu(cyc2ns_offset, i) = offset;

Which would take the funny offset into account and make it resume at
where we left off.

If they got out of sync, we need to flip sched_clock_stable and work on
getting the sched_clock.c code to be monotonic over such a flip.

> So such large values of TSC (leading to a very big difference between
> rq->clock and rq->age_stamp) wont be correctly handled by
> scale_rt_power() either.

Still, we need to fix the clock, not fudge the users.
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