Re: [PATCH 1/3] block, bfq: do not idle if only one cgroup is activated

[yukuai (C)]

On 2021/08/03 15:07, Paolo Valente wrote:
> > Il giorno 31 lug 2021, alle ore 09:10, yukuai (C) <yukuai3@xxxxxxxxxx> ha scritto:
> >
> > On 2021/07/24 15:12, Paolo Valente wrote:
> > > > Il giorno 14 lug 2021, alle ore 11:45, Yu Kuai <yukuai3@xxxxxxxxxx> ha scritto:
> > > >
> > > > If only one group is activated, specifically
> > > > 'bfqd->num_groups_with_pending_reqs == 1', there is no need to guarantee
> > > > the same share of the throughput of queues in the same group.
> > > >
> > > > Thus change the condition from '> 0' to '> 1' in
> > > > bfq_asymmetric_scenario().
> > > I see your point, and I agree with your goal.  Yet, your change seems
> > > not to suffer from the following problem.
> > > In addition to the groups that are created explicitly, there is the
> > > implicit root group.  So, when bfqd->num_groups_with_pending_reqs ==
> > > 1, there may be both active processes in the root group and active
> > > processes in the only group created explicitly.  In this case, idling
> > > is needed to preserve service guarantees.
> > > Probably your idea should be improved by making sure that there is
> > > pending I/O only from either the root group or the explicit group.
> > > Thanks,
> > > Paolo
> >
> >
> > Hi, Paolo
>
> Hi
>
> > I'm trying to add support to judge if root group have pending rqs, the
> > implementation involve setting and clearing the busy state.
>
> I wouldn't use the busy state, as it does not take in-flight requests
> into account.  For I/O control, the latter are as important as the
> ones still queued in the scheduler.  For this reason, I take in-flight
> requests into account when counting
> bfqd->num_groups_with_pending_reqs.
>
> See, e.g., this
>
> 	if (!bfqq->dispatched && !bfq_bfqq_busy(bfqq)) {
> 		...
> 		bfq_weights_tree_remove(bfqd, bfqq);
> 	}
>
> in bfq_completed_request.
>
> I would replicate the same logic in deciding whether the root group
> has pending I/O.

Hi, Paolo

Thanks for your advice, I'll send a new patchset soon.

Kuai