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

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Message-Id: <200712072213.06530.nickpiggin@yahoo.com.au>
Date:	Fri, 7 Dec 2007 22:13:05 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Stefano Brivio <stefano.brivio@...imi.it>,
	Robert Love <rml@...h9.net>, linux-kernel@...r.kernel.org,
	Dave Jones <davej@...hat.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Michael Buesch <mb@...sch.de>,
	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 Friday 07 December 2007 19:45, Ingo Molnar wrote:
> * Stefano Brivio <stefano.brivio@...imi.it> wrote:
> > This patch fixes a regression introduced by:
> >
> > commit bb29ab26863c022743143f27956cc0ca362f258c
> > Author: Ingo Molnar <mingo@...e.hu>
> > Date:   Mon Jul 9 18:51:59 2007 +0200
> >
> > This caused the jiffies counter to leap back and forth on cpufreq
> > changes on my x86 box. I'd say that we can't always assume that TSC
> > does "small errors" only, when marked unstable. On cpufreq changes
> > these errors can be huge.
>
> ah, printk_clock() still uses sched_clock(), not jiffies. So it's not
> the jiffies counter that goes back and forth, it's sched_clock() - so
> this is a printk timestamps anomaly, not related to jiffies. I thought
> we have fixed this bug in the printk code already: sched_clock() is a
> 'raw' interface that should not be used directly - the proper interface
> is cpu_clock(cpu).

It's a single CPU box, so sched_clock() jumping would still be
problematic, no?

My patch should fix the worst cpufreq sched_clock jumping issue
I think.
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