linux-kernel - Re: [PATCH] Fix posix-cpu-timer breakage caused by stale p->last

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Date:	Thu, 22 Feb 2007 10:16:18 +0100
From:	Mike Galbraith <efault@....de>
To:	tglx@...utronix.de
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, john stultz <johnstul@...ibm.com>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Ken Chen <kenneth.w.chen@...el.com>, balducci@...ts.it,
	LKML <linux-kernel@...r.kernel.org>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH] Fix posix-cpu-timer breakage caused by stale
	p->last_ran value

On Thu, 2007-02-22 at 01:33 +0100, Thomas Gleixner wrote:
> Problem description at:
> http://bugzilla.kernel.org/show_bug.cgi?id=8048
> 
> Commit b18ec80396834497933d77b81ec0918519f4e2a7 
>     [PATCH] sched: improve migration accuracy
> optimized the scheduler time calculations, but broke posix-cpu-timers.
> 
> The problem is that the p->last_ran value is not updated after a context
> switch. So a subsequent call to current_sched_time() calculates with a
> stale p->last_ran value, i.e. accounts the full time, which the task was
> scheduled away.

Oops, missed that.  You could also remove the prev->last_ran assignment
just above your addition, and turn this into a negative cost bugfix :)

	-Mike

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