Re: [PATCH 05/10] block: remove per-queue plugging

[Peter Zijlstra]

On Mon, 2011-03-07 at 20:43 +0100, Jens Axboe wrote:
> On 2011-03-07 11:23, Peter Zijlstra wrote:
> > On Sat, 2011-03-05 at 21:54 +0100, Jens Axboe wrote:
> >>
> >> Apparently so. Peter/Ingo, please shoot this one down in flames.
> >> Summary:
> >>
> >> - Need a way to trigger this flushing when a task is going to sleep
> >> - It's currently done right before calling deactivate_task(). We know
> >>   the task is going to sleep here, but it's also under the runqueue
> >>   lock. Not good.
> >> - In the new location, it's not completely clear to me whether we can
> >>   safely deref 'prev' or not. The usage of prev_state would seem to
> >>   indicate that we cannot, and as far as I can tell, prev could at this
> >>   point already potentially be running on another CPU.
> >>
> >> Help? Peter, we talked about this in Tokyo in September. Initial
> >> suggestion was to use preempt notifiers, which we can't because:
> >>
> >> - runqueue lock is also held
> >> - It's not unconditionally available, depends on config.
> >>
> >> diff --git a/kernel/sched.c b/kernel/sched.c
> >> index e806446..8581ad3 100644
> >> --- a/kernel/sched.c
> >> +++ b/kernel/sched.c
> >> @@ -2826,6 +2826,14 @@ static void finish_task_switch(struct rq *rq, struct task_struct *prev)
> >>  #endif /* __ARCH_WANT_INTERRUPTS_ON_CTXSW */
> >>         finish_lock_switch(rq, prev);
> >>  
> >> +       /*
> >> +        * If this task has IO plugged, make sure it
> >> +        * gets flushed out to the devices before we go
> >> +        * to sleep
> >> +        */
> >> +       if (prev_state != TASK_RUNNING)
> >> +               blk_flush_plug(prev);
> >> +
> >>         fire_sched_in_preempt_notifiers(current);
> >>         if (mm)
> >>                 mmdrop(mm);
> >> @@ -3973,14 +3981,6 @@ need_resched_nonpreemptible:
> >>                                 if (to_wakeup)
> >>                                         try_to_wake_up_local(to_wakeup);
> >>                         }
> >> -                       /*
> >> -                        * If this task has IO plugged, make sure it
> >> -                        * gets flushed out to the devices before we go
> >> -                        * to sleep
> >> -                        */
> >> -                       blk_flush_plug(prev);
> >> -                       BUG_ON(prev->plug && !list_empty(&prev->plug->list));
> >> -
> >>                         deactivate_task(rq, prev, DEQUEUE_SLEEP);
> >>                 }
> >>                 switch_count = &prev->nvcsw;
> >>
> > 
> > Right, so your new location is still under rq->lock for a number of
> > architectures (including x86). finish_lock_switch() doesn't actually
> > release the lock unless __ARCH_WANT_INTERRUPTS_ON_CTXSW ||
> > __ARCH_WANT_UNLOCKED_CTXSW (the former implies the latter since rq->lock
> > is IRQ-safe).
> 
> Ah, thanks for that.
> 
> > If you want a safe place to drop rq->lock (but keep in mind to keep IRQs
> > disabled there) and use prev, do something like the below. Both
> > pre_schedule() and idle_balance() can already drop the rq->lock do doing
> > it once more is quite all-right ;-)
> > 
> > Note that once you drop rq->lock prev->state can change to TASK_RUNNING
> > again so don't re-check that.
> 
> So that's a problem. If I end up flushing this structure that sits on
> the stack of the process, I cannot have it running on another CPU at
> that time.
> 
> I need the process to be in such a state that it will not get scheduled
> on another CPU before this has completed.
> 
> Is that even possible? 

Yes, if prev will be flipped back to TASK_RUNNING it will still stay on
that cpu, it will not migrate until the cpu that schedules it away (the
cpu you're on) will have flipped rq->curr, and that happens way after
this point. So you're good to go, just don't rely on ->state once you
release rq->lock.
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