On 6/27/18 1:20 PM, Josef Bacik wrote:
> On Wed, Jun 27, 2018 at 01:06:31PM -0600, Jens Axboe wrote:
>> On 6/25/18 9:12 AM, Josef Bacik wrote:
>>> +static void __blkcg_iolatency_throttle(struct rq_qos *rqos,
>>> + struct iolatency_grp *iolat,
>>> + spinlock_t *lock, bool issue_as_root,
>>> + bool use_memdelay)
>>> + __releases(lock)
>>> + __acquires(lock)
>>> +{
>>> + struct rq_wait *rqw = &iolat->rq_wait;
>>> + unsigned use_delay = atomic_read(&lat_to_blkg(iolat)->use_delay);
>>> + DEFINE_WAIT(wait);
>>> + bool first_block = true;
>>> +
>>> + if (use_delay)
>>> + blkcg_schedule_throttle(rqos->q, use_memdelay);
>>> +
>>> + /*
>>> + * To avoid priority inversions we want to just take a slot if we are
>>> + * issuing as root. If we're being killed off there's no point in
>>> + * delaying things, we may have been killed by OOM so throttling may
>>> + * make recovery take even longer, so just let the IO's through so the
>>> + * task can go away.
>>> + */
>>> + if (issue_as_root || fatal_signal_pending(current)) {
>>> + atomic_inc(&rqw->inflight);
>>> + return;
>>> + }
>>> +
>>> + if (iolatency_may_queue(iolat, &wait, first_block))
>>> + return;
>>> +
>>> + do {
>>> + prepare_to_wait_exclusive(&rqw->wait, &wait,
>>> + TASK_UNINTERRUPTIBLE);
>>> +
>>> + iolatency_may_queue(iolat, &wait, first_block);
>>> + first_block = false;
>>> +
>>> + if (lock) {
>>> + spin_unlock_irq(lock);
>>> + io_schedule();
>>> + spin_lock_irq(lock);
>>> + } else {
>>> + io_schedule();
>>> + }
>>> + } while (1);
>>
>> So how does this wait loop ever exit?
>>
>
> Sigh, I cleaned this up from what we're using in production and did it poorly,
> I'll fix it up. Thanks,
Also may want to consider NOT using exclusive add if first_block == false, as
you'll end up at the tail of the waitqueue after sleeping and being denied.
This is similar to the wbt change I posted last week.
For may_queue(), your wq_has_sleeper() is also going to be always true
inside your loop, since you call it after doing the prepare_to_wait()
which adds you to the queue. That's why wbt does the list checks, but
it'd be nicer to have a wq_has_other_sleepers() for that. So your
first iolatency_may_queue() inside the loop will always be false.
--
Jens Axboe
