> 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.
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
>
