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Subject: Re: [PATCH 12/15] block: introduce blk-iolatency io controller
To:     Josef Bacik <josef@toxicpanda.com>
Cc:     linux-block@vger.kernel.org, kernel-team@fb.com,
        akpm@linux-foundation.org, hannes@cmpxchg.org,
        linux-kernel@vger.kernel.org, tj@kernel.org,
        linux-fsdevel@vger.kernel.org, Josef Bacik <jbacik@fb.com>
References: <20180625151243.2132-1-josef@toxicpanda.com>
 <20180625151243.2132-13-josef@toxicpanda.com>
 <05a581ed-8f21-9d89-a813-a03d802d3469@kernel.dk>
 <20180627192046.ieqncfl6ioy37mof@destiny>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <784f0862-0441-5ed2-1beb-3effa82b3438@kernel.dk>
Date:   Wed, 27 Jun 2018 13:24:55 -0600
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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