linux-kernel - Re: Stop machine threads are getting preemted by the rt period enforcement

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Message-ID: <4846F789.7050702@qualcomm.com>
Date:	Wed, 04 Jun 2008 13:14:01 -0700
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Dimitri Sivanich <sivanich@....com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Nick Piggin <nickpiggin@...oo.com.au>, rostedt@...dmis.org,
	Oleg Nesterov <oleg@...sign.ru>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Paul Menage <menage@...gle.com>,
	"Randy.Dunlap" <rddunlap@...l.org>, suresh.b.siddha@...el.com
Subject: Re: Stop machine threads are getting preemted by the rt	period	enforcement



Peter Zijlstra wrote:
> On Wed, 2008-06-04 at 11:24 -0700, Max Krasnyansky wrote:
>> Peter Zijlstra wrote:
>>> On Wed, 2008-06-04 at 11:07 -0700, Max Krasnyansky wrote:
>>>> Peter, Ingo,
>>>>
>>>> Take a look at the report below (came up during isolcpu= remove discussions).
>>>>
>>>> It looks like stop_machine threads are getting forcefully preempted because
>>>> they exceed their RT quanta. It's strange because rt period is pretty long.
>>>> But given that disabling rt period logic solves the issue the machine was not
>>>> really stuck.
>>> Yeah, I know, I'm already looking at this
>> I see. Does it look like a bug in the rt period logic ?
>> Or did the stop_machine thread really run for a long time (in the report that
>> you got that is) ?
> 
> looks like a fun race between refreshing the period and updating
> cpu_online_map.

Oh, I did not realize that rt period is a timer that iterates online cpus. I
assumed that you do it in the scheduler tick or something.

Max
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