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SubjectRe: [PATCH] cpu hotplug, sched:Introduce cpu_active_map and redoscheddomainmanagment (take 2)
Hi Ingo,
Here is another patch submitted that has not been acked/nacked yet.
If you get a free moment, please let me know your thoughts. Here is the
full thread for your convenience:

http://lkml.org/lkml/2008/7/22/281

(and FYI it was ACKed by Peter here: http://lkml.org/lkml/2008/7/22/286)

-Greg

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@novell.com>
>
> 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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