Re: [PATCH 12/15] block: introduce blk-iolatency io controller

[Josef Bacik <josef@toxicpanda.com>]

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,

Josef