linux-kernel - Re: [PATCH] scheduler: fix x86 regression in native_sched

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Message-Id: <200712081636.45828.mb@bu3sch.de>
Date:	Sat, 8 Dec 2007 16:36:45 +0100
From:	Michael Buesch <mb@...sch.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Mark Lord <lkml@....ca>, Guillaume Chazarain <guichaz@...oo.fr>,
	stefano.brivio@...imi.it, Nick Piggin <nickpiggin@...oo.com.au>,
	Robert Love <rml@...h9.net>, linux-kernel@...r.kernel.org,
	Dave Jones <davej@...hat.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH] scheduler: fix x86 regression in native_sched_clock

On Saturday 08 December 2007 16:33:27 Ingo Molnar wrote:
> 
> * Michael Buesch <mb@...sch.de> wrote:
> 
> > On Saturday 08 December 2007 16:13:41 Ingo Molnar wrote:
> > > 
> > > * Mark Lord <lkml@....ca> wrote:
> > > 
> > > > Ingo Molnar wrote:
> > > >> ...
> > > >> thanks. I do get the impression that most of this can/should wait until 
> > > >> 2.6.25. The patches look quite dangerous.
> > > > ..
> > > >
> > > > I confess to not really trying hard to understand everything in this 
> > > > thread, but the implication seems to be that this bug might affect 
> > > > udelay() and possibly jiffies ?
> > > 
> > > no, it cannot affect jiffies. (jiffies was a red herring all along)
> > > 
> > > udelay() cannot be affected either - sched_clock() has no effect on 
> > > udelay(). _But_, when there are TSC problems then tsc based udelay() 
> > > suffers too so the phenomenons may _seem_ related.
> > 
> > What about msleep()? I suspect problems in b43 because of this issue. 
> > msleep() returning too early. Is that possible with this bug?
> 
> i cannot see how. You can verify msleep by running something like this:
> 
>   while :; do time usleep 111000; done
> 
> you should see a steady stream of:
> 
>  real    0m0.113s
>  real    0m0.113s
>  real    0m0.113s
> 
> (on an idle system). If it fluctuates, with occasional longer delays, 
> there's some timer problem present.

Does the sleeping and timing use different time references?
I mean, if it uses the same reference and that reference does fluctuate
you won't see it in the result.

But anyway, Stefano. Can you test this?

-- 
Greetings Michael.
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