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

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Date:	Tue, 22 Jul 2008 16:16:00 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	Max Krasnyansky <maxk@...lcomm.com>, mingo@...e.hu,
	dmitry.adamushko@...il.com, torvalds@...ux-foundation.org,
	pj@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpu
	hotplug,	sched:Introduce	cpu_active_map	and	redoscheddomainmanagment (take
	2)

On Tue, 2008-07-22 at 10:06 -0400, 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 ;)
> 
> > --
> >
> > 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.
> 
> 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@...ell.com>


Right, makes sense.

ACK

> diff --git a/kernel/sched.c b/kernel/sched.c
> index 62b1b8e..99ba70d 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -6611,7 +6611,7 @@ static void rq_attach_root(struct rq *rq, struct 
> root_domain *rd)
>      rq->rd = rd;
>  
>      cpu_set(rq->cpu, rd->span);
> -    if (cpu_isset(rq->cpu, cpu_online_map))
> +    if (cpu_isset(rq->cpu, cpu_active_map))
>          set_rq_online(rq);
>  
>      spin_unlock_irqrestore(&rq->lock, flags);
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 7f70026..2bae8de 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1004,7 +1004,7 @@ static void yield_task_fair(struct rq *rq)
>   * search starts with cpus closest then further out as needed,
>   * so we always favor a closer, idle cpu.
>   * Domains may include CPUs that are not usable for migration,
> - * hence we need to mask them out (cpu_active_map)
> + * hence we need to mask them out (rq->rd->online)
>   *
>   * Returns the CPU we should wake onto.
>   */
> @@ -1032,7 +1032,7 @@ static int wake_idle(int cpu, struct task_struct *p)
>              || ((sd->flags & SD_WAKE_IDLE_FAR)
>              && !task_hot(p, task_rq(p)->clock, sd))) {
>              cpus_and(tmp, sd->span, p->cpus_allowed);
> -            cpus_and(tmp, tmp, cpu_active_map);
> +            cpus_and(tmp, tmp, task_rq(p)->rd->online);
>              for_each_cpu_mask(i, tmp) {
>                  if (idle_cpu(i)) {
>                      if (i != task_cpu(p)) {
> diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
> index 24621ce..d93169d 100644
> --- a/kernel/sched_rt.c
> +++ b/kernel/sched_rt.c
> @@ -936,13 +936,6 @@ static int find_lowest_rq(struct task_struct *task)
>          return -1; /* No targets found */
>  
>      /*
> -     * Only consider CPUs that are usable for migration.
> -     * I guess we might want to change cpupri_find() to ignore those
> -     * in the first place.
> -     */
> -    cpus_and(*lowest_mask, *lowest_mask, cpu_active_map);
> -
> -    /*
>       * At this point we have built a mask of cpus representing the
>       * lowest priority tasks in the system.  Now we want to elect
>       * the best one based on our affinity and topology.
> 
> --------------
> 
> Regards,
> -Greg
> 

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