linux-kernel - Re: [REPORT] cfs-v4 vs sd-0.44

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Message-ID: <Pine.LNX.4.63.0704232342440.7121@qynat.qvtvafvgr.pbz>
Date:	Mon, 23 Apr 2007 23:45:53 -0700 (PDT)
From:	David Lang <david.lang@...italinsight.com>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Gene Heskett <gene.heskett@...il.com>,
	Peter Williams <pwil3058@...pond.net.au>,
	Arjan van de Ven <arjan@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Juliusz Chroboczek <jch@....jussieu.fr>,
	Con Kolivas <kernel@...ivas.org>, ck list <ck@....kolivas.org>,
	Bill Davidsen <davidsen@....com>, Willy Tarreau <w@....eu>,
	William Lee Irwin III <wli@...omorphy.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr
Subject: Re: [REPORT] cfs-v4 vs sd-0.44

On Tue, 24 Apr 2007, Ingo Molnar wrote:

> * Gene Heskett <gene.heskett@...il.com> wrote:
>
>>> Gene has done some testing under CFS with X reniced to +10 and the
>>> desktop still worked smoothly for him.
>>
>> As a data point here, and probably nothing to do with X, but I did
>> manage to lock it up, solid, reset button time tonight, by wanting
>> 'smart' to get done with an update session after amanda had started.
>> I took both smart processes I could see in htop all the way to -19,
>> but when it was about done about 3 minutes later, everything came to
>> an instant, frozen, reset button required lockup.  I should have
>> stopped at -17 I guess. :(
>
> yeah, i guess this has little to do with X. I think in your scenario it
> might have been smarter to either stop, or to renice the workloads that
> took away CPU power from others to _positive_ nice levels. Negative nice
> levels can indeed be dangerous.
>
> (Btw., to protect against such mishaps in the future i have changed the
> SysRq-N [SysRq-Nice] implementation in my tree to not only change
> real-time tasks to SCHED_OTHER, but to also renice negative nice levels
> back to 0 - this will show up in -v6. That way you'd only have had to
> hit SysRq-N to get the system out of the wedge.)

if you are trying to unwedge a system it may be a good idea to renice all tasks 
to 0, it could be that a task at +19 is holding a lock that something else is 
waiting for.

David Lang
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