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Date: Tue, 7 May 2019 12:25:22 +0100 From: Patrick Bellasi <patrick.bellasi@....com> To: Suren Baghdasaryan <surenb@...gle.com> Cc: LKML <linux-kernel@...r.kernel.org>, linux-pm@...r.kernel.org, linux-api@...r.kernel.org, Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>, "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>, Vincent Guittot <vincent.guittot@...aro.org>, Viresh Kumar <viresh.kumar@...aro.org>, Paul Turner <pjt@...gle.com>, Quentin Perret <quentin.perret@....com>, Dietmar Eggemann <dietmar.eggemann@....com>, Morten Rasmussen <morten.rasmussen@....com>, Juri Lelli <juri.lelli@...hat.com>, Todd Kjos <tkjos@...gle.com>, Joel Fernandes <joelaf@...gle.com>, Steve Muckle <smuckle@...gle.com> Subject: Re: [PATCH v8 08/16] sched/core: uclamp: Set default clamps for RT tasks On 17-Apr 16:07, Suren Baghdasaryan wrote: > On Tue, Apr 2, 2019 at 3:42 AM Patrick Bellasi <patrick.bellasi@....com> wrote: > > > > By default FAIR tasks start without clamps, i.e. neither boosted nor > > capped, and they run at the best frequency matching their utilization > > demand. This default behavior does not fit RT tasks which instead are > > expected to run at the maximum available frequency, if not otherwise > > required by explicitly capping them. > > > > Enforce the correct behavior for RT tasks by setting util_min to max > > whenever: > > > > 1. the task is switched to the RT class and it does not already have a > > user-defined clamp value assigned. > > > > 2. an RT task is forked from a parent with RESET_ON_FORK set. > > > > NOTE: utilization clamp values are cross scheduling class attributes and > > thus they are never changed/reset once a value has been explicitly > > defined from user-space. > > > > Signed-off-by: Patrick Bellasi <patrick.bellasi@....com> > > Cc: Ingo Molnar <mingo@...hat.com> > > Cc: Peter Zijlstra <peterz@...radead.org> > > --- > > kernel/sched/core.c | 26 ++++++++++++++++++++++++++ > > 1 file changed, 26 insertions(+) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index bdebdabe9bc4..71c9dd6487b1 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -1042,6 +1042,28 @@ static int uclamp_validate(struct task_struct *p, > > static void __setscheduler_uclamp(struct task_struct *p, > > const struct sched_attr *attr) > > { > > + unsigned int clamp_id; > > + > > + /* > > + * On scheduling class change, reset to default clamps for tasks > > + * without a task-specific value. > > + */ > > + for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) { > > + struct uclamp_se *uc_se = &p->uclamp_req[clamp_id]; > > + unsigned int clamp_value = uclamp_none(clamp_id); > > + > > + /* Keep using defined clamps across class changes */ > > + if (uc_se->user_defined) > > + continue; > > + > > + /* By default, RT tasks always get 100% boost */ > > + if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN)) > > + clamp_value = uclamp_none(UCLAMP_MAX); > > + > > + uc_se->bucket_id = uclamp_bucket_id(clamp_value); > > + uc_se->value = clamp_value; > > Is it possible for p->uclamp_req[UCLAMP_MAX].value to be less than > uclamp_none(UCLAMP_MAX) for this RT task? If that's a possibility then > I think we will end up with a case of > p->uclamp_req[UCLAMP_MIN].value > p->uclamp_req[UCLAMP_MAX].value > after these assignments are done. > The util_max for an RT task can be less then uclamp_none(UCLAMP_MAX), however, requesting a task specific util_max which is smaller then the current util_min will fail in: __sched_setscheduler() uclamp_validate() since we only allow util_min <= util_max for all task specific values. > > + } > > + > > if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP))) > > return; > > > > @@ -1077,6 +1099,10 @@ static void uclamp_fork(struct task_struct *p) > > for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) { > > unsigned int clamp_value = uclamp_none(clamp_id); > > > > + /* By default, RT tasks always get 100% boost */ > > + if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN)) > > + clamp_value = uclamp_none(UCLAMP_MAX); > > + > > p->uclamp_req[clamp_id].user_defined = false; > > p->uclamp_req[clamp_id].value = clamp_value; > > p->uclamp_req[clamp_id].bucket_id = uclamp_bucket_id(clamp_value); > > -- > > 2.20.1 > > -- #include <best/regards.h> Patrick Bellasi
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