linux-kernel - Re: getcpu after sched

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Date:	Fri, 11 May 2007 09:08:53 +0200
From:	Andi Kleen <ak@...e.de>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	Andi Kleen <ak@...e.de>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: getcpu after sched_setaffinity

On Thu, May 10, 2007 at 04:09:10PM -0700, Ulrich Drepper wrote:
> >is only designed for the case where you don't set the affinity explicitely;
> >otherwise you should already know where you are and don't need it.
> 
> That's not true in general.  Yes, because I want to test vgetcpu() I 
> restrict the set to just one CPU.
> 
> But if I have more than 2 "CPUs" and I set the affinity to two CPUs 
> which currently are not used you cannot make this argument.

All the users I talked to never mentioned this as a use case.
They were only interested in fully unrestricted programs. 

Anyways you can handle it too, just invalidate or do getcpu with 
a new cache afterwards.

Or just wait the jiffie for the cache to expire.

> 
> getcpu should always work correctly not only if you cannot determine it 
> in another way.

getcpu by design is approximate. If you don't understand this
don't bother using it.

-Andi

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