From: Nick Piggin <nickpiggin@yahoo.com.au>
To: "Chen, Kenneth W" <kenneth.w.chen@intel.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: bug in sched.c:task_hot()
Date: Tue, 05 Oct 2004 13:17:55 +1000 [thread overview]
Message-ID: <41621263.2000404@yahoo.com.au> (raw)
In-Reply-To: <200410050237.i952bx620740@unix-os.sc.intel.com>
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
> {
>
>
>-
>To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>the body of a message to majordomo@vger.kernel.org
>More majordomo info at http://vger.kernel.org/majordomo-info.html
>Please read the FAQ at http://www.tux.org/lkml/
>
>
next prev parent reply other threads:[~2004-10-05 3:19 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-10-05 2:38 bug in sched.c:task_hot() Chen, Kenneth W
2004-10-05 3:17 ` Nick Piggin [this message]
2004-10-05 17:15 ` Chen, Kenneth W
2004-10-05 7:33 ` Peter Williams
2004-10-05 7:44 ` Nick Piggin
2004-10-05 8:07 ` Peter Williams
2004-10-05 8:42 ` Nick Piggin
2004-10-05 10:03 ` Peter Williams
2004-10-05 17:39 ` Chen, Kenneth W
2004-10-05 22:09 ` Peter Williams
2004-10-05 8:03 ` Ingo Molnar
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=41621263.2000404@yahoo.com.au \
--to=nickpiggin@yahoo.com.au \
--cc=kenneth.w.chen@intel.com \
--cc=linux-kernel@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox