Re: [PATCH BUGFIX/IMPROVEMENT 2/6] block, bfq: put reqs of waker and woken in dispatch list

[Jan Kara]

On Thu 28-01-21 18:54:05, Paolo Valente wrote:
> 
> 
> > Il giorno 26 gen 2021, alle ore 17:18, Jens Axboe <axboe@xxxxxxxxx> ha scritto:
> > 
> > On 1/26/21 3:50 AM, Paolo Valente wrote:
> >> Consider a new I/O request that arrives for a bfq_queue bfqq. If, when
> >> this happens, the only active bfq_queues are bfqq and either its waker
> >> bfq_queue or one of its woken bfq_queues, then there is no point in
> >> queueing this new I/O request in bfqq for service. In fact, the
> >> in-service queue and bfqq agree on serving this new I/O request as
> >> soon as possible. So this commit puts this new I/O request directly
> >> into the dispatch list.
> >> 
> >> Tested-by: Jan Kara <jack@xxxxxxx>
> >> Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
> >> ---
> >> block/bfq-iosched.c | 17 ++++++++++++++++-
> >> 1 file changed, 16 insertions(+), 1 deletion(-)
> >> 
> >> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> >> index a83149407336..e5b83910fbe0 100644
> >> --- a/block/bfq-iosched.c
> >> +++ b/block/bfq-iosched.c
> >> @@ -5640,7 +5640,22 @@ static void bfq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
> >> 
> >> 	spin_lock_irq(&bfqd->lock);
> >> 	bfqq = bfq_init_rq(rq);
> >> -	if (!bfqq || at_head || blk_rq_is_passthrough(rq)) {
> >> +
> >> +	/*
> >> +	 * Additional case for putting rq directly into the dispatch
> >> +	 * queue: the only active bfq_queues are bfqq and either its
> >> +	 * waker bfq_queue or one of its woken bfq_queues. In this
> >> +	 * case, there is no point in queueing rq in bfqq for
> >> +	 * service. In fact, the in-service queue and bfqq agree on
> >> +	 * serving this new I/O request as soon as possible.
> >> +	 */
> >> +	if (!bfqq ||
> >> +	    (bfqq != bfqd->in_service_queue &&
> >> +	     bfqd->in_service_queue != NULL &&
> >> +	     bfq_tot_busy_queues(bfqd) == 1 + bfq_bfqq_busy(bfqq) &&
> >> +	     (bfqq->waker_bfqq == bfqd->in_service_queue ||
> >> +	      bfqd->in_service_queue->waker_bfqq == bfqq)) ||
> >> +	    at_head || blk_rq_is_passthrough(rq)) {
> >> 		if (at_head)
> >> 			list_add(&rq->queuelist, &bfqd->dispatch);
> >> 		else
> >> 
> > 
> > This is unreadable... Just seems like you are piling heuristics in to
> > catch some case, and it's neither readable nor clean.
> > 
> 
> Yeah, these comments inappropriately assume that the reader knows the
> waker mechanism in depth.  And they do not stress at all how important
> this improvement is.
> 
> I'll do my best to improve these comments.
> 
> To try to do a better job, let me also explain the matter early here.
> Maybe you or others can give me some early feedback (or just tell me
> to proceed).
> 
> This change is one of the main improvements that boosted
> throughput in Jan's tests.  Here is the rationale:
> - consider a bfq_queue, say Q1, detected as a waker of another
>   bfq_queue, say Q2
> - by definition of a waker, Q1 blocks the I/O of Q2, i.e., some I/O of
>   of Q1 needs to be completed for new I/O of Q1 to arrive.  A notable
					       ^^ Q2?

>   example is journald
> - so, Q1 and Q2 are in any respect two cooperating processes: if the
>   service of Q1's I/O is delayed, Q2 can only suffer from it.
>   Conversely, if Q2's I/O is delayed, the purpose of Q1 is just defeated.

What do you exactly mean by this last sentence?

> - as a consequence if some I/O of Q1/Q2 arrives while Q2/Q1 is the
>   only queue in service, there is absolutely no point in delaying the
>   service of such an I/O.  The only possible result is a throughput
>   loss, detected by Jan's test

If we are idling at that moment waiting for more IO from in service queue,
I agree. But that doesn't seem to be part of your condition above?

> - so, when the above condition holds, the most effective and efficient
>   action is to put the new I/O directly in the dispatch list
> - as an additional restriction, Q1 and Q2 must be the only busy queues
>   for this commit to put the I/O of Q2/Q1 in the dispatch list.  This is
>   necessary, because, if also other queues are waiting for service, then
>   putting new I/O directly in the dispatch list may evidently cause a
>   violation of service guarantees for the other queues

This last restriction is not ideal for cases like jbd2 thread since it may
still lead to pointless idling but I understand that without some
restriction like this several waking threads could just starve other ones.
So I guess it's fine for now.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR