linux-kernel - Re: is minimum udelay() not respected in preemptible SMP kernel-2.6.23?

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Message-ID: <4732F728.8020707@qumranet.com>
Date:	Thu, 08 Nov 2007 13:46:48 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Andi Kleen <ak@...e.de>
CC:	Matt Mackall <mpm@...enic.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marin Mitov <mitov@...p.bas.bg>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: is minimum udelay() not respected in preemptible SMP kernel-2.6.23?

Andi Kleen wrote:
> On Thursday 08 November 2007 01:20, Matt Mackall wrote:
>   
>> On Wed, Nov 07, 2007 at 12:30:45PM -0800, Andrew Morton wrote:
>>     
>>> Ow.  Yes, from my reading delay_tsc() can return early (or after
>>> heat-death-of-the-universe) if the TSCs are offset and if preemption
>>> migrates the calling task between CPUs.
>>>
>>> I suppose a lameo fix would be to disable preemption in delay_tsc().
>>>       
>> preempt_disable is lousy documentation here. This and other cases
>> (lots of per_cpu users, IIRC) actually want a migrate_disable() which
>> is a proper subset. We can simply implement migrate_disable() as
>> preempt_disable() for now and come back later and implement a proper
>> migrate_disable() that still allows preemption (and thus avoids the
>> latency).
>>     
>
> We could actually do this right now. migrate_disable() can be just changing
> the cpu affinity of the current thread to current cpu and then restoring it 
> afterwards. That should even work from interrupt context.
>
> get_cpu() etc. could be changed to use this then too.
>
>   

What if some other thread calls sched_setaffinity() on the 
migrate_disable()d cpu?  we'd need to detect this to avoid 
migrate_enable() stomping on sched_setaffinity()'s work.


-- 
error compiling committee.c: too many arguments to function

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