linux-kernel - Re: CPU POSIX timers livelock

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Message-ID: <20080502172803.GA1541@tv-sign.ru>
Date:	Fri, 2 May 2008 21:28:03 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Bj?rn Steinbrink <B.Steinbrink@....de>
Cc:	Petr Tesarik <ptesarik@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Roland McGrath <roland@...hat.com>,
	Frank Mayhar <fmayhar@...gle.com>
Subject: Re: CPU POSIX timers livelock

On 05/02, Bj?rn Steinbrink wrote:
>
> Hm, I have some message in that thread from April they don't seem to be
> on lkml.org nor in the bugzilla though. Oh well, I can send you an mbox
> of that thread in a private mail, just let me know if you want that.

Me too please ;)

Please note there is a related problem if we have a single thread but
a lot of timers attached to this thread. If the signal is ignored, each
timer should be re-armed, this all can consume >= 1/HZ CPU time.
We have "maxfire = 20" in check_thread_timers/check_process_timers to
prevent this, but it is just a workaround. And it is a bit silly to just
waste the CPU if the signal is ignored, arm_timer() is heavy and doesn't
scale well.

However, I have already forgot this code, perhaps I missed something,
and at least maxfire does help.

Oleg.

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