Messages in this thread | | | Date | Thu, 1 Jul 2021 15:20:50 +0000 | From | Quentin Perret <> | Subject | Re: [PATCH v3 1/3] sched: Fix UCLAMP_FLAG_IDLE setting |
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On Thursday 01 Jul 2021 at 15:57:50 (+0100), Qais Yousef wrote: > On 07/01/21 12:43, Quentin Perret wrote: > > On Thursday 01 Jul 2021 at 12:08:03 (+0100), Qais Yousef wrote: > > > On 07/01/21 10:07, Quentin Perret wrote: > > > > On Wednesday 30 Jun 2021 at 15:45:14 (+0000), Quentin Perret wrote: > > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > > > > index b094da4c5fea..c0b999a8062a 100644 > > > > > --- a/kernel/sched/core.c > > > > > +++ b/kernel/sched/core.c > > > > > @@ -980,7 +980,6 @@ static inline void uclamp_idle_reset(struct rq *rq, enum uclamp_id clamp_id, > > > > > if (!(rq->uclamp_flags & UCLAMP_FLAG_IDLE)) > > > > > return; > > > > > > > > > > - rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE; > > > > > WRITE_ONCE(rq->uclamp[clamp_id].value, clamp_value); > > > > > } > > > > > > > > > > @@ -1253,6 +1252,10 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p) > > > > > > > > > > for_each_clamp_id(clamp_id) > > > > > uclamp_rq_inc_id(rq, p, clamp_id); > > > > > + > > > > > + /* Reset clamp idle holding when there is one RUNNABLE task */ > > > > > + if (rq->uclamp_flags & UCLAMP_FLAG_IDLE) > > > > > + rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE; > > > > > } > > > > > > > > > > static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p) > > > > > @@ -1300,6 +1303,13 @@ uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id) > > > > > if (p->uclamp[clamp_id].active) { > > > > > uclamp_rq_dec_id(rq, p, clamp_id); > > > > > uclamp_rq_inc_id(rq, p, clamp_id); > > > > > + > > > > > + /* > > > > > + * Make sure to clear the idle flag if we've transiently reached > > > > > + * 0 uclamp active tasks on the rq. > > > > > + */ > > > > > + if (rq->uclamp_flags & UCLAMP_FLAG_IDLE) > > > > > + rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE; > > > > > > > > Bah, now that I had coffee I realize this has the exact same problem. > > > > Let me look at this again ... > > > > > > Hehe uclamp has this effect. It's all obvious, until it's not :-) > > > > Indeed ... :) > > > > > Yes this needs to be out of the loop. > > > > Right or maybe we can just check that uclamp_id == UCLAMP_MAX here and > > we should be good to go? That is, what about the below? > > Wouldn't it be better to do this from uclamp_idle_reset() then?
That should work too, but clearing the flag outside of uclamp_rq_inc_id() feels a little bit more robust to ordering issues.
Specifically, uclamp_rq_inc() has the following pattern:
for_each_clamp_id(clamp_id) uclamp_rq_inc_id(rq, p , clamp_id);
if (rq->uclamp_flags & UCLAMP_FLAG_IDLE) rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
So, if we change this to clear the flag from uclamp_rq_inc_id()->uclamp_idle_reset() then we'll have issues if (for example) for_each_clamp_id()'s order changes in the future. IOW, it feels cleaner to not create side effects in uclamp_rq_inc_id() that impact the idle flag given that its very own behaviour depends on the flag.
WDYT?
Cheers, Quentin
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