linux-kernel - Re: getcpu after sched

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Message-id: <46459400.1000900@informatik.uni-halle.de>
Date:	Sat, 12 May 2007 12:16:32 +0200
From:	Bert Wesarg <wesarg@...ormatik.uni-halle.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

Hello,

(Please CC me, thank you)

Ulrich Drepper wrote:
> The attached test program fails on a dual core (and probably SMP) 
> machine on x86-64.  Depending on where the thread starts, in one of the 
> iterations the sched_setffinity() call succeeds but then sched_getcpu() 
> fails to report the correct CPU.
> 
> In set_cpus_allowed migrate_task() is called if the new CPU set does not 
> include the current CPU.  I hope that migrate_task() also works for 
> p==current.
> 
> This leaves the x86-64 vgetcpu() implementation as the weak point.  Is 
> the caching causing problems?  Should migrate_task() make sure the cache 
> is reset?
> 
> You need a very recent glibc to compile (glibc-2.5.90-22 in rawhide). 
> If this is not available replace the sched_getcpu() call.  But make sure 
> you pass a pointer to a cache.
> 
> 
A question not related to this problem, but may I ask why the libc don't
use[1] the linux/getcpu.h header to get the opaque struct vgetcpu_cache?

Thank You

Bert Wesarg

[1]: http://sourceware.org/ml/glibc-cvs/2007-q2/msg00098.html
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