linux-kernel - getcpu after sched

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Message-ID: <46439BBA.2070705@redhat.com>
Date:	Thu, 10 May 2007 15:24:58 -0700
From:	Ulrich Drepper <drepper@...hat.com>
To:	Andi Kleen <ak@...e.de>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: getcpu after sched_setaffinity

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.

-- 
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖

View attachment "tst-getcpu.c" of type "text/plain" (875 bytes)