Messages in this thread | | | Date | Thu, 1 Jul 2021 12:32:32 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH v2] sched/uclamp: Avoid getting unreasonable ucalmp value when rq is idle |
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On 06/30/21 22:12, Xuewen Yan wrote: > From: Xuewen Yan <xuewen.yan@unisoc.com> > > Now in uclamp_rq_util_with(), when the task != NULL, the uclamp_max as following: > uc_rq_max = rq->uclamp[UCLAMP_MAX].value; > uc_eff_max = uclamp_eff_value(p, UCLAMP_MAX); > uclamp_max = max{uc_rq_max, uc_eff_max}; > > Consider the following scenario: > (1)the rq is idle, the uc_rq_max is last runnable task's UCLAMP_MAX; > (2)the p's uc_eff_max < uc_rq_max. > > As a result, the uclamp_max = uc_rq_max instead of uc_eff_max, it is unreasonable. > > The scenario often happens in find_energy_efficient_cpu(), when the task has smaller UCLAMP_MAX. > > When rq has UCLAMP_FLAG_IDLE flag, enqueuing the task will lift UCLAMP_FLAG_IDLE > and set the rq clamp as the task's via uclamp_idle_reset(). It doesn't need > to read the rq clamp. And it can also avoid the problems described above. > > Fixes: 9d20ad7dfc9a ("sched/uclamp: Add uclamp_util_with()") > > Signed-off-by: Xuewen Yan <xuewen.yan@unisoc.com> > > --- > change v2: > *add Fixes(Valentin Schneider); > *ignore all rq clamp when idle (Valentin Schneider) > --- > kernel/sched/sched.h | 21 ++++++++++++++------- > 1 file changed, 14 insertions(+), 7 deletions(-) > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index c80d42e9589b..14a41a243f7b 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -2818,20 +2818,27 @@ static __always_inline > unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util, > struct task_struct *p) > { > - unsigned long min_util; > - unsigned long max_util; > + unsigned long min_util = 0; > + unsigned long max_util = 0; > > if (!static_branch_likely(&sched_uclamp_used)) > return util; > > - min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value); > - max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value); > - > if (p) { > - min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN)); > - max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX)); > + min_util = uclamp_eff_value(p, UCLAMP_MIN); > + max_util = uclamp_eff_value(p, UCLAMP_MAX); > + > + /* > + * Ignore last runnable task's max clamp, as this task will > + * reset it. Similarly, no need to read the rq's min clamp. > + */ > + if (rq->uclamp_flags & UCLAMP_FLAG_IDLE) > + goto out;
We read rq->uclamp_flags without locks here. Me thinks this needs READ_ONCE(). But since we care only about a single bit, I can't see any risk for inconsistency, so we're fine.
Reviewed-by: Qais Yousef <qais.yousef@arm.com>
Thanks!
-- Qais Yousef
> } > > + min_util = max_t(unsigned long, min_util, READ_ONCE(rq->uclamp[UCLAMP_MIN].value)); > + max_util = max_t(unsigned long, max_util, READ_ONCE(rq->uclamp[UCLAMP_MAX].value)); > +out: > /* > * Since CPU's {min,max}_util clamps are MAX aggregated considering > * RUNNABLE tasks with _different_ clamps, we can end up with an > -- > 2.25.1 >
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