Re: [PATCH -next v7 2/3] block, bfq: refactor the counting of 'num_groups_with_pending_reqs'

[Yu Kuai]

在 2022/05/30 16:10, Paolo Valente 写道:
> > Il giorno 28 mag 2022, alle ore 11:50, Yu Kuai <yukuai3@xxxxxxxxxx> ha scritto:
> >
> > Currently, bfq can't handle sync io concurrently as long as they
> > are not issued from root group. This is because
> > 'bfqd->num_groups_with_pending_reqs > 0' is always true in
> > bfq_asymmetric_scenario().
> >
> > The way that bfqg is counted into 'num_groups_with_pending_reqs':
> >
> > Before this patch:
> > 1) root group will never be counted.
> > 2) Count if bfqg or it's child bfqgs have pending requests.
> > 3) Don't count if bfqg and it's child bfqgs complete all the requests.
> >
> > After this patch:
> > 1) root group is counted.
> > 2) Count if bfqg have at least one bfqq that is marked busy.
> > 3) Don't count if bfqg doesn't have any busy bfqqs.
>
> Unfortunately, I see a last problem here. I see a double change:
> (1) a bfqg is now counted only as a function of the state of its child
>      queues, and not of also its child bfqgs
> (2) the state considered for counting a bfqg moves from having pending
>      requests to having busy queues
>
> I'm ok with with (1), which is a good catch (you are lady explained
> the idea to me some time ago IIRC).
>
> Yet I fear that (2) is not ok.  A bfqq can become non busy even if it
> still has in-flight I/O, i.e.  I/O being served in the drive.  The
> weight of such a bfqq must still be considered in the weights_tree,
> and the group containing such a queue must still be counted when
> checking whether the scenario is asymmetric.  Otherwise service
> guarantees are broken.  The reason is that, if a scenario is deemed as
> symmetric because in-flight I/O is not taken into account, then idling
> will not be performed to protect some bfqq, and in-flight I/O may
> steal bandwidth to that bfqq in an uncontrolled way.
Hi, Paolo

Thanks for your explanation.

My orginal thoughts was using weights_tree insertion/removal, however,
Jan convinced me that using bfq_add/del_bfqq_busy() is ok.

From what I see, when bfqq dispatch the last request,
bfq_del_bfqq_busy() will not be called from __bfq_bfqq_expire() if
idling is needed, and it will delayed to when such bfqq get scheduled as
in-service queue again. Which means the weight of such bfqq should still
be considered in the weights_tree.

I also run some tests on null_blk with "irqmode=2
completion_nsec=100000000(100ms) hw_queue_depth=1", and tests show
that service guarantees are still preserved on slow device.

Do you this is strong enough to cover your concern?

Thanks,
Kuai
> I verified this also experimentally a few years ago, when I added this
> weights_tree stuff.  That's the rationale behind the part of
> bfq_weights_tree_remove that this patch eliminates.  IOW,
> for a bfqq and its parent bfqg to be out of the count for symmetry,
> all bfqq's requests must also be completed.
>
> Thanks,
> Paolo
>
> > The main reason to use busy state of bfqq instead of 'pending requests'
> > is that bfqq can stay busy after dispatching the last request if idling
> > is needed for service guarantees.
> >
> > With this change, the occasion that only one group is activated can be
> > detected, and next patch will support concurrent sync io in the
> > occasion.
> >
> > This patch also rename 'num_groups_with_pending_reqs' to
> > 'num_groups_with_busy_queues'.
> >
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > Reviewed-by: Jan Kara <jack@xxxxxxx>
> > ---
> > block/bfq-iosched.c | 46 ++-----------------------------------
> > block/bfq-iosched.h | 55 ++++++---------------------------------------
> > block/bfq-wf2q.c    | 19 ++++------------
> > 3 files changed, 13 insertions(+), 107 deletions(-)
> >
> > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> > index 0d46cb728bbf..eb1da1bd5eb4 100644
> > --- a/block/bfq-iosched.c
> > +++ b/block/bfq-iosched.c
> > @@ -852,7 +852,7 @@ static bool bfq_asymmetric_scenario(struct bfq_data *bfqd,
> >
> > 	return varied_queue_weights || multiple_classes_busy
> > #ifdef CONFIG_BFQ_GROUP_IOSCHED
> > -	       || bfqd->num_groups_with_pending_reqs > 0
> > +	       || bfqd->num_groups_with_busy_queues > 0
> > #endif
> > 		;
> > }
> > @@ -970,48 +970,6 @@ void __bfq_weights_tree_remove(struct bfq_data *bfqd,
> > void bfq_weights_tree_remove(struct bfq_data *bfqd,
> > 			     struct bfq_queue *bfqq)
> > {
> > -	struct bfq_entity *entity = bfqq->entity.parent;
> > -
> > -	for_each_entity(entity) {
> > -		struct bfq_sched_data *sd = entity->my_sched_data;
> > -
> > -		if (sd->next_in_service || sd->in_service_entity) {
> > -			/*
> > -			 * entity is still active, because either
> > -			 * next_in_service or in_service_entity is not
> > -			 * NULL (see the comments on the definition of
> > -			 * next_in_service for details on why
> > -			 * in_service_entity must be checked too).
> > -			 *
> > -			 * As a consequence, its parent entities are
> > -			 * active as well, and thus this loop must
> > -			 * stop here.
> > -			 */
> > -			break;
> > -		}
> > -
> > -		/*
> > -		 * The decrement of num_groups_with_pending_reqs is
> > -		 * not performed immediately upon the deactivation of
> > -		 * entity, but it is delayed to when it also happens
> > -		 * that the first leaf descendant bfqq of entity gets
> > -		 * all its pending requests completed. The following
> > -		 * instructions perform this delayed decrement, if
> > -		 * needed. See the comments on
> > -		 * num_groups_with_pending_reqs for details.
> > -		 */
> > -		if (entity->in_groups_with_pending_reqs) {
> > -			entity->in_groups_with_pending_reqs = false;
> > -			bfqd->num_groups_with_pending_reqs--;
> > -		}
> > -	}
> > -
> > -	/*
> > -	 * Next function is invoked last, because it causes bfqq to be
> > -	 * freed if the following holds: bfqq is not in service and
> > -	 * has no dispatched request. DO NOT use bfqq after the next
> > -	 * function invocation.
> > -	 */
> > 	__bfq_weights_tree_remove(bfqd, bfqq,
> > 				  &bfqd->queue_weights_tree);
> > }
> > @@ -7118,7 +7076,7 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
> > 	bfqd->idle_slice_timer.function = bfq_idle_slice_timer;
> >
> > 	bfqd->queue_weights_tree = RB_ROOT_CACHED;
> > -	bfqd->num_groups_with_pending_reqs = 0;
> > +	bfqd->num_groups_with_busy_queues = 0;
> >
> > 	INIT_LIST_HEAD(&bfqd->active_list);
> > 	INIT_LIST_HEAD(&bfqd->idle_list);
> > diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
> > index d92adbdd70ee..6c6cd984d769 100644
> > --- a/block/bfq-iosched.h
> > +++ b/block/bfq-iosched.h
> > @@ -197,9 +197,6 @@ struct bfq_entity {
> > 	/* flag, set to request a weight, ioprio or ioprio_class change  */
> > 	int prio_changed;
> >
> > -	/* flag, set if the entity is counted in groups_with_pending_reqs */
> > -	bool in_groups_with_pending_reqs;
> > -
> > 	/* last child queue of entity created (for non-leaf entities) */
> > 	struct bfq_queue *last_bfqq_created;
> > };
> > @@ -496,52 +493,14 @@ struct bfq_data {
> > 	struct rb_root_cached queue_weights_tree;
> >
> > 	/*
> > -	 * Number of groups with at least one descendant process that
> > -	 * has at least one request waiting for completion. Note that
> > -	 * this accounts for also requests already dispatched, but not
> > -	 * yet completed. Therefore this number of groups may differ
> > -	 * (be larger) than the number of active groups, as a group is
> > -	 * considered active only if its corresponding entity has
> > -	 * descendant queues with at least one request queued. This
> > -	 * number is used to decide whether a scenario is symmetric.
> > -	 * For a detailed explanation see comments on the computation
> > -	 * of the variable asymmetric_scenario in the function
> > -	 * bfq_better_to_idle().
> > -	 *
> > -	 * However, it is hard to compute this number exactly, for
> > -	 * groups with multiple descendant processes. Consider a group
> > -	 * that is inactive, i.e., that has no descendant process with
> > -	 * pending I/O inside BFQ queues. Then suppose that
> > -	 * num_groups_with_pending_reqs is still accounting for this
> > -	 * group, because the group has descendant processes with some
> > -	 * I/O request still in flight. num_groups_with_pending_reqs
> > -	 * should be decremented when the in-flight request of the
> > -	 * last descendant process is finally completed (assuming that
> > -	 * nothing else has changed for the group in the meantime, in
> > -	 * terms of composition of the group and active/inactive state of child
> > -	 * groups and processes). To accomplish this, an additional
> > -	 * pending-request counter must be added to entities, and must
> > -	 * be updated correctly. To avoid this additional field and operations,
> > -	 * we resort to the following tradeoff between simplicity and
> > -	 * accuracy: for an inactive group that is still counted in
> > -	 * num_groups_with_pending_reqs, we decrement
> > -	 * num_groups_with_pending_reqs when the first descendant
> > -	 * process of the group remains with no request waiting for
> > -	 * completion.
> > -	 *
> > -	 * Even this simpler decrement strategy requires a little
> > -	 * carefulness: to avoid multiple decrements, we flag a group,
> > -	 * more precisely an entity representing a group, as still
> > -	 * counted in num_groups_with_pending_reqs when it becomes
> > -	 * inactive. Then, when the first descendant queue of the
> > -	 * entity remains with no request waiting for completion,
> > -	 * num_groups_with_pending_reqs is decremented, and this flag
> > -	 * is reset. After this flag is reset for the entity,
> > -	 * num_groups_with_pending_reqs won't be decremented any
> > -	 * longer in case a new descendant queue of the entity remains
> > -	 * with no request waiting for completion.
> > +	 * Number of groups with at least one bfqq that is marked busy,
> > +	 * and this number is used to decide whether a scenario is symmetric.
> > +	 * Note that bfqq is busy doesn't mean that the bfqq contains requests.
> > +	 * If idling is needed for service guarantees, bfqq will stay busy
> > +	 * after dispatching the last request, see details in
> > +	 * __bfq_bfqq_expire().
> > 	 */
> > -	unsigned int num_groups_with_pending_reqs;
> > +	unsigned int num_groups_with_busy_queues;
> >
> > 	/*
> > 	 * Per-class (RT, BE, IDLE) number of bfq_queues containing
> > diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
> > index b97e33688335..48ca7922035c 100644
> > --- a/block/bfq-wf2q.c
> > +++ b/block/bfq-wf2q.c
> > @@ -221,13 +221,15 @@ static bool bfq_no_longer_next_in_service(struct bfq_entity *entity)
> > static void bfq_inc_busy_queues(struct bfq_queue *bfqq)
> > {
> > 	bfqq->bfqd->busy_queues[bfqq->ioprio_class - 1]++;
> > -	bfqq_group(bfqq)->busy_queues++;
> > +	if (!(bfqq_group(bfqq)->busy_queues++))
> > +		bfqq->bfqd->num_groups_with_busy_queues++;
> > }
> >
> > static void bfq_dec_busy_queues(struct bfq_queue *bfqq)
> > {
> > 	bfqq->bfqd->busy_queues[bfqq->ioprio_class - 1]--;
> > -	bfqq_group(bfqq)->busy_queues--;
> > +	if (!(--bfqq_group(bfqq)->busy_queues))
> > +		bfqq->bfqd->num_groups_with_busy_queues--;
> > }
> >
> > #else /* CONFIG_BFQ_GROUP_IOSCHED */
> > @@ -1006,19 +1008,6 @@ static void __bfq_activate_entity(struct bfq_entity *entity,
> > 		entity->on_st_or_in_serv = true;
> > 	}
> >
> > -#ifdef CONFIG_BFQ_GROUP_IOSCHED
> > -	if (!bfq_entity_to_bfqq(entity)) { /* bfq_group */
> > -		struct bfq_group *bfqg =
> > -			container_of(entity, struct bfq_group, entity);
> > -		struct bfq_data *bfqd = bfqg->bfqd;
> > -
> > -		if (!entity->in_groups_with_pending_reqs) {
> > -			entity->in_groups_with_pending_reqs = true;
> > -			bfqd->num_groups_with_pending_reqs++;
> > -		}
> > -	}
> > -#endif
> > -
> > 	bfq_update_fin_time_enqueue(entity, st, backshifted);
> > }
> >
> > --
> > 2.31.1
>
> .