Message-ID: <41621263.2000404@yahoo.com.au>
Date: Tue, 05 Oct 2004 13:17:55 +1000
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: "Chen, Kenneth W" <kenneth.w.chen@intel.com>
CC: linux-kernel@vger.kernel.org
Subject: Re: bug in sched.c:task_hot()
References: <200410050237.i952bx620740@unix-os.sc.intel.com>
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Chen, Kenneth W wrote:

>Current implementation of task_hot() has a performance bug in it
>that it will cause integer underflow.
>
>Variable "now" (typically passed in as rq->timestamp_last_tick)
>and p->timestamp are all defined as unsigned long long.  However,
>If former is smaller than the latter, integer under flow occurs
>which make the result of subtraction a huge positive number. Then
>it is compared to sd->cache_hot_time and it will wrongly identify
>a cache hot task as cache cold.
>
>This bug causes large amount of incorrect process migration across
>cpus (at stunning 10,000 per second) and we lost cache affinity very
>quickly and almost took double digit performance regression on a db
>transaction processing workload.  Patch to fix the bug.  Diff'ed against
>2.6.9-rc3.
>
>Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
>

This one looks OK (the other may need a bit of rethinking).
What kernel is the regression in relation to, out of interest?

>
>--- linux-2.6.9-rc3/kernel/sched.c.orig	2004-10-04 19:11:21.000000000 -0700
>+++ linux-2.6.9-rc3/kernel/sched.c	2004-10-04 19:19:27.000000000 -0700
>@@ -180,7 +180,8 @@ static unsigned int task_timeslice(task_
> 	else
> 		return SCALE_PRIO(DEF_TIMESLICE, p->static_prio);
> }
>-#define task_hot(p, now, sd) ((now) - (p)->timestamp < (sd)->cache_hot_time)
>+#define task_hot(p, now, sd) ((long long) ((now) - (p)->timestamp)	\
>+				< (long long) (sd)->cache_hot_time)
>
> enum idle_type
> {
>
>
>-
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