Re: [Xen-devel] [PATCH] xen: sched_credit: filter node-affinity mask against online cpus

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 16/09/13 16:23, Dario Faggioli wrote:
> On lun, 2013-09-16 at 15:00 +0100, George Dunlap wrote:
> > On 13/09/13 17:09, Dario Faggioli wrote:
> >
> > > diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
> > > index dbe6de6..d76cd88 100644
> > > --- a/xen/common/sched_credit.c
> > > +++ b/xen/common/sched_credit.c
> > > @@ -627,10 +638,18 @@ _csched_cpu_pick(const struct scheduler *ops, struct vcpu 
> > > *vc, bool_t commit)
> > >        int balance_step;
> > >    
> > >        online = cpupool_scheduler_cpumask(vc->domain->cpupool);
> > > +    cpumask_and(&cpus, vc->cpu_affinity, online);
> > > +
> > >        for_each_csched_balance_step( balance_step )
> > >        {
> > > +        /*
> > > +         * We filter the node-affinity mask against
> > > +         * [vc->cpu_affinity && online] here to avoid problems if all
> > > +         * the cpus in in the node-affinity mask are offline (e.g.,
> > > +         * because the domain moved to a different cpupool).
> > > +         */
> > It's probably worth mentioning that the reason you do the cpumask_and
> > here and not in the other place this is called is that cpumask_cycle is
> > called after this one (which will choke on an empy mask), but not in the
> > other place it's called.
> Well, it is indeed what the comment says, or at leas what I wanted it to
> say. :-)
>
> I tied to concentrate on the actual issue, which is avoid picking an
> offline cpu, independently on which specific ASSERT is the one that
> triggers.
Ok, but the question is, why AND it with online cpus here, but not in 
the other place?  If the "actual issue" is that the mask may be empty, 
it should be done in both places.  If the "actual issue" is that 
cpu_cycle() chokes on empty masks, then that's an important think to 
point out, so poor programmers don't get confused as to why it's 
different. :-)
 -George

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