On 6/28/18 7:26 AM, Josef Bacik wrote:
> 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.
I missed that you don't do finish_wait() in the loop, I had played with that
to see if it fixes things. But yeah, as it stands, you are right.
>> 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,
Basically we need to do the same thing in wbt and blk-iolatency for this,
so we should sync them up.
--
Jens Axboe
