Re: 2.6.16 - sys_sched_getaffinity & hotplug

[Nathan Lynch <ntl@pobox.com>]

Paul Jackson 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);
> 
> Adding Robert Love to the cc list, as he is Mr. sched_getaffinity,
> I believe.
> 
> I ended up doing a similar change, to the cpus (and mems) masks
> in the root (all encompassing) cpuset.

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.

> These now show the values
> of cpu_online_map and node_online_map, not *_MASK_ALL.
> 
> My hunches are:
>  * This change to cpu_online_map is a good one.

It's not.

>  * The man page sentence "Usually, all bits in the mask are set."
>    might have meant something when it was written, but it is not
>    now clear what.

I think it could reasonably be interpreted as all bits in the mask are
set unless the task's affinity has been modified.