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