On Wed, Jun 27, 2018 at 01:24:55PM -0600, Jens Axboe wrote:
> 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.
>
This isn't how it works though. You aren't removed from the list until you do
finish_wait(), so you don't lose your spot on the list. We only get added to
the end of the list if
if (list_empty(&wq_entry->entry))
otherwise nothing changes.
> 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.
Ah yeah that's a good point, I'll go back to using what you had to catch that
case. Thanks,
Josef
