Date: Fri, 27 Jan 2006 23:23:56 -0600
From: Nathan Lynch <ntl@pobox.com>
To: Paul Jackson <pj@sgi.com>
Cc: steiner@sgi.com, mingo@elte.hu, linux-kernel@vger.kernel.org,
       rml@novell.com
Subject: Re: 2.6.16 - sys_sched_getaffinity & hotplug
Message-ID: <20060128052355.GC18730@localhost.localdomain>
References: <20060127230659.GA4752@sgi.com> <20060127191400.aacb8539.pj@sgi.com> <20060128034241.GB18730@localhost.localdomain> <20060127205834.b5821a02.pj@sgi.com>
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Paul Jackson wrote:
> Nathan wrote:
> > Which is problematic, because cpuset_cpus_allowed ->
> > guarantee_online_cpus restricts the task->cpus_allowed mask to cpus
> > which happen to be online at the time of the call to
> > sched_setaffinity.  If more cpus come online later, that task can't be
> > migrated to them.
> 
> Well, sort of.
> 
> A task could always migrate - just because a sched_getaffinity
> the task did in the past doesn't show a CPU as valid, doesn't stop
> the task from asking to pin to that CPU now.

I was speaking of the setaffinity (not getaffinity) case -- I assumed
this was what you were referring to since I couldn't find any calls to
the cpuset code in the getaffinity path.


> One of three lessons could be taken from your example:
>  1) return all possible CPUS (CPU_MASK_ALL, likely), as you
> recommend

I'm only recommending not changing the current behavior of
sched_getaffinity.

(BTW - cpu_possible_map can be a subset of CPU_MASK_ALL on some
platforms -- powerpc, at least, since we can discover the number of
truly possible cpus early in boot.)

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