Re: [PATCH -next v10 3/4] block, bfq: refactor the counting of 'num_groups_with_pending_reqs'

[Yu Kuai]

Hi, Paolo!

在 2022/09/06 17:37, Paolo Valente 写道:
> > Il giorno 26 ago 2022, alle ore 04:34, Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> ha scritto:
> >
> > Hi, Paolo!
> >
> > 在 2022/08/25 22:59, Paolo Valente 写道:
> > > > Il giorno 11 ago 2022, alle ore 03:19, Yu Kuai <yukuai1@xxxxxxxxxxxxxxx <mailto:yukuai1@xxxxxxxxxxxxxxx>> ha scritto:
> > > >
> > > > Hi, Paolo
> > > >
> > > > 在 2022/08/10 18:49, Paolo Valente 写道:
> > > > > > Il giorno 27 lug 2022, alle ore 14:11, Yu Kuai <yukuai1@xxxxxxxxxxxxxxx <mailto:yukuai1@xxxxxxxxxxxxxxx>> ha scritto:
> > > > > >
> > > > > > Hi, Paolo
> > > > > hi
> > > > > > Are you still interested in this patchset?
> > > > > Yes. Sorry for replying very late again.
> > > > > Probably the last fix that you suggest is enough, but I'm a little bit
> > > > > concerned that it may be a little hasty.  In fact, before this fix, we
> > > > > exchanged several messages, and I didn't seem to be very good at
> > > > > convincing you about the need to keep into account also in-service
> > > > > I/O.  So, my question is: are you sure that now you have a
> > > >
> > > > I'm confused here, I'm pretty aware that in-service I/O(as said pending
> > > > requests is the patchset) should be counted, as you suggested in v7, are
> > > > you still thinking that the way in this patchset is problematic?
> > > >
> > > > I'll try to explain again that how to track is bfqq has pending pending
> > > > requests, please let me know if you still think there are some problems:
> > > >
> > > > patch 1 support to track if bfqq has pending requests, it's
> > > > done by setting the flag 'entity->in_groups_with_pending_reqs' when the
> > > > first request is inserted to bfqq, and it's cleared when the last
> > > > request is completed. specifically the flag is set in
> > > > bfq_add_bfqq_busy() when 'bfqq->dispatched' if false, and it's cleared
> > > > both in bfq_completed_request() and bfq_del_bfqq_busy() when
> > > > 'bfqq->diapatched' is false.
> > > This general description seems correct to me. Have you already sent a new version of your patchset?
> >
> > It's glad that we finially on the same page here.
>
> Yep. Sorry for my chronicle delay.

Better late than never 😁
> > Please take a look at patch 1, which already impelement the above
> > descriptions, it seems to me there is no need to send a new version
> > for now. If you think there are still some other problems, please let
> > me know.
>
> Patch 1 seems ok to me. I seem to have only one pending comment on this patch (3/4) instead. Let me paste previous stuff here for your convenience:
That sounds good.

> > > -	/*
> > > -	 * 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.
> > > -	 */
> > I would really love it if you leave this comment.  I added it after
> > suffering a lot for a nasty UAF.  Of course the first sentence may
> > need to be adjusted if the code that precedes it is to be removed.
> > Same as above, if this patch is applied, this function will be gone.
>
> yes, but this comment now must be moved forward.
>
> Looking forward for a new complete version, for a new review.  I'll do
> my best to reply quicker.
I'll send a new version soon, perhaps I'll also add the following
cleanups to this patchset.

Thanks,
Kuai
> Thanks,
> Paolo
>
>
>
>
>
> > Thanks,
> > Kuai
> > > Thanks,
> > > Paolo
> > > > Thanks,
> > > > Kuai
> > > > > clear/complete understanding of this non-trivial matter?
> > > > > Consequently, are we sure that this last fix is most certainly all we
> > > > > need?  Of course, I will check on my own, but if you reassure me on
> > > > > this point, I will feel more confident.
> > > > > Thanks,
> > > > > Paolo
> > > > > > 在 2022/07/20 19:38, Yu Kuai 写道:
> > > > > > > Hi
> > > > > > >
> > > > > > > 在 2022/07/20 19:24, Paolo VALENTE 写道:
> > > > > > > > > Il giorno 12 lug 2022, alle ore 15:30, Yu Kuai <yukuai1@xxxxxxxxxxxxxxx <mailto:yukuai1@xxxxxxxxxxxxxxx> <mailto:yukuai1@xxxxxxxxxxxxxxx>> ha scritto:
> > > > > > > > >
> > > > > > > > > Hi!
> > > > > > > > >
> > > > > > > > > I'm copying my reply with new mail address, because Paolo seems
> > > > > > > > > didn't receive my reply.
> > > > > > > > >
> > > > > > > > > 在 2022/06/23 23:32, Paolo Valente 写道:
> > > > > > > > > > Sorry for the delay.
> > > > > > > > > > > Il giorno 10 giu 2022, alle ore 04:17, Yu Kuai <yukuai3@xxxxxxxxxx <mailto:yukuai3@xxxxxxxxxx> <mailto: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 pending requests.
> > > > > > > > > > > 3) Don't count if bfqg complete all the requests.
> > > > > > > > > > >
> > > > > > > > > > > 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.
> > > > > > > > > > >
> > > > > > > > > > > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx <mailto:yukuai3@xxxxxxxxxx> <mailto:yukuai3@xxxxxxxxxx>>
> > > > > > > > > > > Reviewed-by: Jan Kara <jack@xxxxxxx <mailto:jack@xxxxxxx> <mailto:jack@xxxxxxx>>
> > > > > > > > > > > ---
> > > > > > > > > > > block/bfq-iosched.c | 42 ------------------------------------------
> > > > > > > > > > > block/bfq-iosched.h | 18 +++++++++---------
> > > > > > > > > > > block/bfq-wf2q.c    | 19 ++++---------------
> > > > > > > > > > > 3 files changed, 13 insertions(+), 66 deletions(-)
> > > > > > > > > > >
> > > > > > > > > > > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> > > > > > > > > > > index 0ec21018daba..03b04892440c 100644
> > > > > > > > > > > --- a/block/bfq-iosched.c
> > > > > > > > > > > +++ b/block/bfq-iosched.c
> > > > > > > > > > > @@ -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--;
> > > > > > > > > > > -}
> > > > > > > > > > > -}
> > > > > > > > > > With this part removed, I'm missing how you handle the following
> > > > > > > > > > sequence of events:
> > > > > > > > > > 1.  a queue Q becomes non busy but still has dispatched requests, so
> > > > > > > > > > it must not be removed from the counter of queues with pending reqs
> > > > > > > > > > yet
> > > > > > > > > > 2.  the last request of Q is completed with Q being still idle (non
> > > > > > > > > > busy).  At this point Q must be removed from the counter.  It seems to
> > > > > > > > > > me that this case is not handled any longer
> > > > > > > > > Hi, Paolo
> > > > > > > > >
> > > > > > > > > 1) At first, patch 1 support to track if bfqq has pending requests, it's
> > > > > > > > > done by setting the flag 'entity->in_groups_with_pending_reqs' when the
> > > > > > > > > first request is inserted to bfqq, and it's cleared when the last
> > > > > > > > > request is completed(based on weights_tree insertion and removal).
> > > > > > > >
> > > > > > > > In patch 1 I don't see the flag cleared for the request-completion event :(
> > > > > > > >
> > > > > > > > The piece of code involved is this:
> > > > > > > >
> > > > > > > > static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd)
> > > > > > > > {
> > > > > > > > u64 now_ns;
> > > > > > > > u32 delta_us;
> > > > > > > >
> > > > > > > > bfq_update_hw_tag(bfqd);
> > > > > > > >
> > > > > > > > bfqd->rq_in_driver[bfqq->actuator_idx]--;
> > > > > > > > bfqd->tot_rq_in_driver--;
> > > > > > > > bfqq->dispatched--;
> > > > > > > >
> > > > > > > > if (!bfqq->dispatched && !bfq_bfqq_busy(bfqq)) {
> > > > > > > > /*
> > > > > > > > * Set budget_timeout (which we overload to store the
> > > > > > > > * time at which the queue remains with no backlog and
> > > > > > > > * no outstanding request; used by the weight-raising
> > > > > > > > * mechanism).
> > > > > > > > */
> > > > > > > > bfqq->budget_timeout = jiffies;
> > > > > > > >
> > > > > > > > bfq_weights_tree_remove(bfqd, bfqq);
> > > > > > > > }
> > > > > > > > ...
> > > > > > > >
> > > > > > > > Am I missing something?
> > > > > > >
> > > > > > > I add a new api bfq_del_bfqq_in_groups_with_pending_reqs() in patch 1
> > > > > > > to clear the flag, and it's called both from bfq_del_bfqq_busy() and
> > > > > > > bfq_completed_request(). I think you may miss the later:
> > > > > > >
> > > > > > > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> > > > > > > index 0d46cb728bbf..0ec21018daba 100644
> > > > > > > --- a/block/bfq-iosched.c
> > > > > > > +++ b/block/bfq-iosched.c
> > > > > > > @@ -6263,6 +6263,7 @@ static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd)
> > > > > > >            */
> > > > > > >           bfqq->budget_timeout = jiffies;
> > > > > > >
> > > > > > > +        bfq_del_bfqq_in_groups_with_pending_reqs(bfqq);
> > > > > > >           bfq_weights_tree_remove(bfqd, bfqq);
> > > > > > >       }
> > > > > > >
> > > > > > > Thanks,
> > > > > > > Kuai
> > > > > > > > Thanks,
> > > > > > > > Paolo
> > > > > .
>
> .