linux-kernel - Re: [Bug #23902] 2.6.37-rc3 massive interactivity regression on ARM

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Date:	Wed, 25 May 2011 09:56:51 -0500
From:	"Steven J. Magnani" <steve@...idescorp.com>
To:	andi@...stfloor.org, LKML <linux-kernel@...r.kernel.org>
Cc:	rjw@...k.pl, stable@...nel.org, a.p.zijlstra@...llo.nl
Subject: Re: [Bug #23902] 2.6.37-rc3 massive interactivity regression on ARM

Andi -

This bug appears to have crept into 2.6.35.12 via
a3fe22ee824895aafdc1b788e19c081a2e6dd9da and is still present in
2.6.35.13.

Mainline patch fe44d62122829959e960bc699318d58966922a69 (sched: Fix the
irqtime code to deal with u64 wraps) seems to fix my system although I
recommend also applying  8e92c20183ed0579d94501311b81c42b65cb2129
(sched: Fix the irqtime code for 32bit). Any stable releases that have
picked up 305e6835e05513406fa12820e40e4a8ecb63743c (sched: Do not
account irq time to current task) should probably also apply these.

Before I could get these patches to apply to 2.6.35.13 I had to tweak
sched.c:update_rq_clock() to match
f26f9aff6aaf67e9a430d16c266f91b13a5bff64 (Sched: fix skip_clock_update
optimization). That patch was applied to 2.6.35-longterm, except for the
update_rq_clock() portion. I believe that was because that portion
depended on a patch from Ventakesh that wasn't applied until later. It
looks like the logic that ended up in 2.6.35.12 was slightly different
than in mainline; I'm not familiar enough with the scheduler details to
know what the effects would be.

------------------------------------------------------------------------
Signed-off-by: Steven J. Magnani <steve@...idescorp.com>
---
diff -uprN a/kernel/sched.c b/kernel/sched.c
--- a/kernel/sched.c	2011-05-25 09:39:05.200320708 -0500
+++ b/kernel/sched.c	2011-05-25 09:39:27.990238065 -0500
@@ -654,8 +654,10 @@ inline void update_rq_clock(struct rq *r
 	int cpu = cpu_of(rq);
 	u64 irq_time;

-	if (!rq->skip_clock_update)
-		rq->clock = sched_clock_cpu(cpu_of(rq));
+	if (rq->skip_clock_update)
+		return;
+
+	rq->clock = sched_clock_cpu(cpu);
 	irq_time = irq_time_cpu(cpu);
 	if (rq->clock - irq_time > rq->clock_task)
 		rq->clock_task = rq->clock - irq_time;
------------------------------------------------------------------------
 Steven J. Magnani               "I claim this network for MARS!
 www.digidescorp.com              Earthling, return my space modulator!"

 #include <standard.disclaimer>

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