linux-kernel - Re: x86: disable preemption in delay

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Message-ID: <473D5016.4000105@qumranet.com>
Date:	Fri, 16 Nov 2007 10:08:54 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	akpm@...ux-foundation.org, mingo@...e.hu
Subject: Re: x86: disable preemption in delay_tsc()

Arjan van de Ven wrote:
> On Thu, 15 Nov 2007 04:00:47 GMT
> Linux Kernel Mailing List <linux-kernel@...r.kernel.org> wrote:
>
>   
>> Gitweb:
>> http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=35d5d08a085c56f153458c3f5d8ce24123617faf
>> Commit:     35d5d08a085c56f153458c3f5d8ce24123617faf Parent:
>> 7eea436433b7b18045f272562e256976f593f7c0 Author:     Andrew Morton
>> <akpm@...ux-foundation.org> AuthorDate: Wed Nov 14 17:00:41 2007 -0800
>> Committer:  Linus Torvalds <torvalds@...dy.linux-foundation.org>
>> CommitDate: Wed Nov 14 18:45:44 2007 -0800
>>
>>     x86: disable preemption in delay_tsc()
>>     
>>     Marin Mitov points out that delay_tsc() can misbehave if it is
>> preempted and rescheduled on a different CPU which has a skewed TSC.
>> Fix it by disabling preemption.
>>  
>>     
>
> this worries me.. this appears to effectively disable preemption during
> udelay() and mdelay() loops... which are very obvious latency inducers.
>
> Now you can argue that if you're preemptible you should have used
> msleep() and co, and I'll totally buy that. 
>
>
> Maybe we should just check if we're still on the same cpu or something,
> or have a cheap way to pin a process to a cpu.... but both are longer
> term solutions.
>
>   

You can use preemption notifiers to get a callback when you are
preempted.  Not sure what you'd to with that callback, though.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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