Date: Sat, 28 Jan 2006 13:27:36 -0600
From: Nathan Lynch <ntl@pobox.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Paul Jackson <pj@sgi.com>, Jack Steiner <steiner@sgi.com>,
       linux-kernel@vger.kernel.org, Robert Love <rml@novell.com>,
       Andrew Morton <akpm@osdl.org>
Subject: Re: 2.6.16 - sys_sched_getaffinity & hotplug
Message-ID: <20060128192736.GD18730@localhost.localdomain>
References: <20060127230659.GA4752@sgi.com> <20060127191400.aacb8539.pj@sgi.com> <20060128133244.GA22704@elte.hu>
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Ingo Molnar wrote:
> 
> > Jack wrote:
> > > Should the following change be made to sched_getaffinity(). 
> > > 
> > > Index: linux/kernel/sched.c
> > > ===================================================================
> > > --- linux.orig/kernel/sched.c	2006-01-25 08:50:21.401747695 -0600
> > > +++ linux/kernel/sched.c	2006-01-27 16:57:24.504871895 -0600
> > > @@ -4031,7 +4031,7 @@ long sched_getaffinity(pid_t pid, cpumas
> > >  		goto out_unlock;
> > >  
> > >  	retval = 0;
> > > -	cpus_and(*mask, p->cpus_allowed, cpu_possible_map);
> > > +	cpus_and(*mask, p->cpus_allowed, cpu_online_map);
> > 
> In any case, Jack's change looks reasonable and obviously correct.

Are you sure?  Assuming this change is in effect, consider the
following:

Task starts with default affinity.

Task does sched_getaffinity, stashes the result in saved_mask.

Task pins itself to one cpu and does some work.

Meanwhile, more cpus are brought online.

Task finishes work and does sched_setaffinity(saved_mask).

Task will never run on the new cpus.


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