linux-kernel - Re: 2.6.35-rc3: System unresponsive under load

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Date:	Wed, 30 Jun 2010 21:07:54 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Luca Tettamanti <kronos.it@...il.com>
CC:	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Julia Lawall <julia@...u.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	maciej.rutecki@...il.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: 2.6.35-rc3: System unresponsive under load

Hi Luca,

On 06/26/2010 06:47 PM, Luca Tettamanti wrote:
>
> Confirmed here: your test program freezes the system for a while under
> 2.6.35-rc3, while vanilla 2.6.34 copes fine.
> sysrq-t was responsive during the freeze, so I took a snapshot during
> it, file is attached.
>
>    
Ignore my test program:
If the master thread is interrupted in the right place, then there are 
400 runnable tasks in the runqueue.
It seems that the scheduler just processes these 400 tasks first instead 
of the keventd/ksoftirqd that is necessary for the keyboard handling.

Attached is a new idea, could you try it with your httpd test?

Perhaps the race is actually a race in the user space:
The exit path of semtimedop() does not contain an explicit memory barrier.
For the kernel, it does not matter: It merely reads one integer value.
If sysret is also no memory barrier, then user space might observe stale 
data.

Which cpu do you have? I was unable to show any misbehavior on a Phenom X4.

--
     Manfred

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