Re: [PATCH] sched: Do not stop cpu in set_cpus_allowed_ptr() if task is not running

[Kirill Tkhai]

Hi, Preeti,

Ð ÐÐ, 15/09/2014 Ð 15:18 +0530, Preeti Murthy ÐÐÑÐÑ:
> Hi Kirill,
> 
> Which tree is this patch based on? __migrate_task() does a
> double_rq_lock/unlock() today in mainline, doesn't it? I don't
> however see that in your patch.

it's based on recent tip tree:
https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/

Also, once I received a build-robot message about my patch
makes warning on allyesconfig, and since that time I learned
about Peter's tree :) 

https://git.kernel.org/cgit/linux/kernel/git/peterz/queue.git

(This is for info, I don't know if it is right to make patches
based on in. But really good if you was away for some time
and you're interested in recent news without lkml archive reading.
I use it :)

Regards,
Kirill

> On Fri, Sep 12, 2014 at 4:33 PM, Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
> >
> > If a task is queued but not running on it rq, we can simply migrate
> > it without migration thread and switching of context.
> >
> > Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> > ---
> >  kernel/sched/core.c |   47 ++++++++++++++++++++++++++++++++---------------
> >  1 file changed, 32 insertions(+), 15 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index d4399b4..dbbba26 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4594,6 +4594,33 @@ void init_idle(struct task_struct *idle, int cpu)
> >  }
> >
> >  #ifdef CONFIG_SMP
> > +/*
> > + * move_queued_task - move a queued task to new rq.
> > + *
> > + * Returns (locked) new rq. Old rq's lock is released.
> > + */
> > +static struct rq *move_queued_task(struct task_struct *p, int new_cpu)
> > +{
> > +       struct rq *rq = task_rq(p);
> > +
> > +       lockdep_assert_held(&rq->lock);
> > +
> > +       dequeue_task(rq, p, 0);
> > +       p->on_rq = TASK_ON_RQ_MIGRATING;
> > +       set_task_cpu(p, new_cpu);
> > +       raw_spin_unlock(&rq->lock);
> > +
> > +       rq = cpu_rq(new_cpu);
> > +
> > +       raw_spin_lock(&rq->lock);
> > +       BUG_ON(task_cpu(p) != new_cpu);
> > +       p->on_rq = TASK_ON_RQ_QUEUED;
> > +       enqueue_task(rq, p, 0);
> > +       check_preempt_curr(rq, p, 0);
> > +
> > +       return rq;
> > +}
> > +
> >  void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
> >  {
> >         if (p->sched_class && p->sched_class->set_cpus_allowed)
> > @@ -4650,14 +4677,15 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
> >                 goto out;
> >
> >         dest_cpu = cpumask_any_and(cpu_active_mask, new_mask);
> > -       if (task_on_rq_queued(p) || p->state == TASK_WAKING) {
> > +       if (task_running(rq, p) || p->state == TASK_WAKING) {
> >                 struct migration_arg arg = { p, dest_cpu };
> >                 /* Need help from migration thread: drop lock and wait. */
> >                 task_rq_unlock(rq, p, &flags);
> >                 stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
> >                 tlb_migrate_finish(p->mm);
> >                 return 0;
> > -       }
> > +       } else if (task_on_rq_queued(p))
> > +               rq = move_queued_task(p, dest_cpu);
> >  out:
> >         task_rq_unlock(rq, p, &flags);
> >
> > @@ -4700,19 +4728,8 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
> >          * If we're not on a rq, the next wake-up will ensure we're
> >          * placed properly.
> >          */
> > -       if (task_on_rq_queued(p)) {
> > -               dequeue_task(rq, p, 0);
> > -               p->on_rq = TASK_ON_RQ_MIGRATING;
> > -               set_task_cpu(p, dest_cpu);
> > -               raw_spin_unlock(&rq->lock);
> > -
> > -               rq = cpu_rq(dest_cpu);
> > -               raw_spin_lock(&rq->lock);
> > -               BUG_ON(task_rq(p) != rq);
> > -               p->on_rq = TASK_ON_RQ_QUEUED;
> > -               enqueue_task(rq, p, 0);
> > -               check_preempt_curr(rq, p, 0);
> > -       }
> > +       if (task_on_rq_queued(p))
> > +               rq = move_queued_task(p, dest_cpu);
> >  done:
> >         ret = 1;
> >  fail:
> >
> >
> >
> > --
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