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

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Message-Id: <1172104409.25076.165.camel@localhost.localdomain>
Date:	Thu, 22 Feb 2007 01:33:29 +0100
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, john stultz <johnstul@...ibm.com>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Mike Galbraith <efault@....de>,
	Ken Chen <kenneth.w.chen@...el.com>, balducci@...ts.it,
	LKML <linux-kernel@...r.kernel.org>, Greg KH <greg@...ah.com>
Subject: [PATCH] Fix posix-cpu-timer breakage caused by stale p->last_ran
	value

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.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -3566,7 +3566,7 @@ switch_tasks:
 
 	sched_info_switch(prev, next);
 	if (likely(prev != next)) {
-		next->timestamp = now;
+		next->timestamp = next->last_ran = now;
 		rq->nr_switches++;
 		rq->curr = next;
 		++*switch_count;



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