linux-kernel - Re: [PATCH 2/4] stop_machine: reimplement using cpuhog

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Message-ID: <4B958ABC.3010401@kernel.org>
Date:	Tue, 09 Mar 2010 08:39:40 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
CC:	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	rusty@...tcorp.com.au, sivanich@....com,
	torvalds@...ux-foundation.org, mingo@...e.hu, peterz@...radead.org,
	dipankar@...ibm.com, josh@...edesktop.org,
	paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 2/4] stop_machine: reimplement using cpuhog

Hello,

On 03/09/2010 04:37 AM, Heiko Carstens wrote:
>> How cpuhog can make a difference? Afaics, we shouldn't pass a
>> blocking callback to hog_cpus/hog_one_cpu.
> 
> Well, it might me true that this shouldn't be done. But I don't see
> a reason why in general it wouldn't work to pass a function that
> would block.  So it's just a matter of time until somebody uses it
> for such a purpose.  For the current stop_machine implementation it
> would be broken to pass a blocking function (preemption disabled,
> interrupts disabled).

Well, all current users don't block and it definitely can be enforced
by turning off preemption around the callback.  stop_machine() uses
busy waiting for every state transition so something else blocking on
a cpu could waste a lot of cpu cycles on other cpus even if the wait
is guaranteed to be finite.  Would that sooth your concern?

Thanks.

-- 
tejun
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