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

[Jens Axboe]

On 2011-03-07 21:41, Peter Zijlstra wrote:
> 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.

Great, that'll work for me! Your patch should work as-is, then. Thanks
Peter.

-- 
Jens Axboe

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