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

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Date:	Wed, 18 Aug 2010 17:20:13 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 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.

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.

thanks,
suresh

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