Re: [PATCH] cpu hotplug, sched:Introduce cpu_active_map and redoscheddomainmanagment(take 2)

[Max Krasnyansky]

Gregory Haskins wrote:
> Max Krasnyansky wrote:
> > Greg, correct me if I'm wrong but we seem to have exact same issue 
> > with the rq->rq->online map. Lets take "cpu going down" for
> > example. We're clearing rq->rd->online bit on DYING event, but
> > nothing AFAICS prevents another cpu calling
> > rebuild_sched_domains()->partition_sched_domains() in the middle of
> > the hotplug sequence. partition_sched_domains() will happily reset
> > rd->rq->online mask and things will fail. I'm talking about this
> > path
> >
> > __build_sched_domains() -> cpu_attach_domain() -> rq_attach_root()
> >     ...
> >     cpu_set(rq->cpu, rd->span);
> >     if (cpu_isset(rq->cpu, cpu_online_map))
> >         set_rq_online(rq);
> >     ...
> >
> >   
>
> I think you are right, but wouldn't s/online/active above fix that as 
> well?  The active_map didnt exist at the time that code went in 
> initially ;)

Actually after a bit more thinking :) I realized that the scenario I 
explained above cannot happen because partition_sched_domains() must be 
called under get_online_cpus() and the set_rq_online() happens in the 
hotplug writer's path (ie under cpu_hotplug.lock). Since I unified all 
the other domain rebuild paths (arch_reinit_sched_domains, etc) we 
should be safe. But it again means we'd rely on those intricate 
dependencies that we wanted to avoid with the cpu_active_map. Also 
cpusets might still need to rebuild the domains in the hotplug writer's 
path.
So it's better to fix it once and for all :)

> > --
> >
> > btw Why didn't we convert sched*.c to use rq->rd->online when it was
> > introduced ? ie Instead of using cpu_online_map directly.
> >   
> I think things were converted where they made sense to convert.  But we 
> also had a different goal at that time, so perhaps something was 
> missed.  If you think something else should be converted, please point 
> it out.
Ok. I'll keep an eye on it.

> In the meantime, I would suggest we consider this patch on top of yours 
> (applies to tip/sched/devel):
>
> ----------------------
>
> sched: Fully integrate cpus_active_map and root-domain code
>   Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>

Ack.
The only thing I'm a bit unsure of is the error scenarios in the cpu 
hotplug event sequence. online_map is not cleared when something in the 
notifier chain fails, but active_map is.

Max
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