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From: Patrick Bellasi <patrick.bellasi@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
	Ingo Molnar <mingo@redhat.com>, Tejun Heo <tj@kernel.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Paul Turner <pjt@google.com>,
	Quentin Perret <quentin.perret@arm.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Juri Lelli <juri.lelli@redhat.com>, Todd Kjos <tkjos@google.com>,
	Joel Fernandes <joelaf@google.com>,
	Steve Muckle <smuckle@google.com>,
	Suren Baghdasaryan <surenb@google.com>
Subject: Re: [PATCH v4 06/16] sched/cpufreq: uclamp: add utilization clamping for FAIR tasks
Date: Fri, 14 Sep 2018 14:57:12 +0100
Message-ID: <20180914135712.GQ1413@e110439-lin> (raw)
In-Reply-To: <20180914133654.GL24124@hirez.programming.kicks-ass.net>

On 14-Sep 15:36, Peter Zijlstra wrote:
> On Fri, Sep 14, 2018 at 02:19:19PM +0100, Patrick Bellasi wrote:
> > On 14-Sep 11:32, Peter Zijlstra wrote:
> 
> > > Should that not be:
> > > 
> > > 	util = clamp_util(rq, cpu_util_cfs(rq));
> > > 
> > > Because if !util might we not still want to enforce the min clamp?
> > 
> > If !util CFS tasks should have been gone since a long time
> > (proportional to their estimated utilization) and thus it probably
> > makes sense to not affect further energy efficiency for tasks of other
> > classes.
> 
> I don't remember what we do for util for new tasks; but weren't we
> talking about setting that to 0 recently? IIRC the problem was that if
> we start at 1 with util we'll always run new tasks on big cores, or
> something along those lines.

Mmm.. could have been in a recent discussion with Quentin, but I
think I've missed it. I know we have something similar on Android for
similar reasons.

> So new tasks would still trigger this case until they'd accrued enough
> history.

Well, yes and no. New tasks will be clamped which means that if they
are generated from a capped parent (or within a cgroups with a
suitable util_max) they can still live in a smaller capacity CPU
despite their utilization being 1024. Thus, to a certain extend,
UtilClamp could be a fix for the above misbehavior whenever needed.

NOTE: this series does not include tasks biasing bits.

> Either way around, I don't much care at this point except I think it
> would be good to have a comment to record the assumptions.

Sure, will add a comment on that and a warning about possible side
effects on tasks placement

> > > Would that not be more readable as:
> > > 
> > > static inline unsigned int uclamp_value(struct rq *rq, int clamp_id)
> > > {
> > > 	unsigned int val = rq->uclamp.value[clamp_id];
> > > 
> > > 	if (unlikely(val == UCLAMP_NOT_VALID))
> > > 		val = uclamp_none(clamp_id);
> > > 
> > > 	return val;
> > > }
> > 
> > I'm trying to keep consistency in variable names usages by always
> > accessing the rq's clamps via a *uc_cpu to make it easy grepping the
> > code. Does this argument make sense ?
> > 
> > On the other side, what you propose above is more easy to read
> > by looking just at that function.... so, if you prefer it better, I'll
> > update it on v5.
> 
> I prefer my version, also because it has a single load of the value (yes
> I know about CSE passes). I figure one can always grep for uclamp or
> something.

+1

> > > And how come NOT_VALID is possible? I thought the idea was to always
> > > have all things a valid value.
> > 
> > When we update the CPU's clamp for a "newly idle" CPU, there are not
> > tasks refcounting clamps and thus we end up with UCLAMP_NOT_VALID for
> > that CPU. That's how uclamp_cpu_update() is currently encoded.
> > 
> > Perhaps we can set the value to uclamp_none(clamp_id) from that
> > function, but I was thinking that perhaps it could be useful to track
> > explicitly that the CPU is now idle.
> 
> IIRC you added an explicit flag to track idle somewhere.. to keep the
> last max clamp in effect or something.

Right... that patch was after this one on v3, but know that I've moved
it before we can probably simplify this path.

> I think, but haven't overly thought about this, that if you always
> ensure these things are valid you can avoid a bunch of NOT_VALID
> conditions. And less conditions is always good, right? :-)

Right, will check better all the usages and remove them when not
strictly required.

Cheers,
Patrick

-- 
#include <best/regards.h>

Patrick Bellasi

  reply index

Thread overview: 80+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-28 13:53 [PATCH v4 00/16] Add utilization clamping support Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 01/16] sched/core: uclamp: extend sched_setattr to support utilization clamping Patrick Bellasi
2018-09-05 11:01   ` Juri Lelli
2018-08-28 13:53 ` [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups Patrick Bellasi
2018-09-05 10:45   ` Juri Lelli
2018-09-06 13:48     ` Patrick Bellasi
2018-09-06 14:13       ` Juri Lelli
2018-09-06  8:17   ` Juri Lelli
2018-09-06 14:00     ` Patrick Bellasi
2018-09-08 23:47   ` Suren Baghdasaryan
2018-09-12 10:32     ` Patrick Bellasi
2018-09-12 13:49   ` Peter Zijlstra
2018-09-12 15:56     ` Patrick Bellasi
2018-09-12 16:12       ` Peter Zijlstra
2018-09-12 17:35         ` Patrick Bellasi
2018-09-12 17:42           ` Peter Zijlstra
2018-09-12 17:52             ` Patrick Bellasi
2018-09-13 19:14               ` Peter Zijlstra
2018-09-14  8:51                 ` Patrick Bellasi
2018-09-12 16:24   ` Peter Zijlstra
2018-09-12 17:42     ` Patrick Bellasi
2018-09-13 19:20       ` Peter Zijlstra
2018-09-14  8:47         ` Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 03/16] sched/core: uclamp: add CPU's clamp groups accounting Patrick Bellasi
2018-09-12 17:34   ` Peter Zijlstra
2018-09-12 17:44     ` Patrick Bellasi
2018-09-13 19:12   ` Peter Zijlstra
2018-09-14  9:07     ` Patrick Bellasi
2018-09-14 11:52       ` Peter Zijlstra
2018-09-14 13:41         ` Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 04/16] sched/core: uclamp: update CPU's refcount on clamp changes Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 05/16] sched/core: uclamp: enforce last task UCLAMP_MAX Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 06/16] sched/cpufreq: uclamp: add utilization clamping for FAIR tasks Patrick Bellasi
2018-09-14  9:32   ` Peter Zijlstra
2018-09-14 13:19     ` Patrick Bellasi
2018-09-14 13:36       ` Peter Zijlstra
2018-09-14 13:57         ` Patrick Bellasi [this message]
2018-09-27 10:23           ` Quentin Perret
2018-08-28 13:53 ` [PATCH v4 07/16] sched/core: uclamp: extend cpu's cgroup controller Patrick Bellasi
2018-08-28 18:29   ` Randy Dunlap
2018-08-29  8:53     ` Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 08/16] sched/core: uclamp: propagate parent clamps Patrick Bellasi
2018-09-09  3:02   ` Suren Baghdasaryan
2018-09-12 12:51     ` Patrick Bellasi
2018-09-12 15:56       ` Suren Baghdasaryan
2018-09-11 15:18   ` Tejun Heo
2018-09-11 16:26     ` Patrick Bellasi
2018-09-11 16:28       ` Tejun Heo
2018-08-28 13:53 ` [PATCH v4 09/16] sched/core: uclamp: map TG's clamp values into CPU's clamp groups Patrick Bellasi
2018-09-09 18:52   ` Suren Baghdasaryan
2018-09-12 14:19     ` Patrick Bellasi
2018-09-12 15:53       ` Suren Baghdasaryan
2018-08-28 13:53 ` [PATCH v4 10/16] sched/core: uclamp: use TG's clamps to restrict Task's clamps Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 11/16] sched/core: uclamp: add system default clamps Patrick Bellasi
2018-09-10 16:20   ` Suren Baghdasaryan
2018-09-11 16:46     ` Patrick Bellasi
2018-09-11 19:25       ` Suren Baghdasaryan
2018-08-28 13:53 ` [PATCH v4 12/16] sched/core: uclamp: update CPU's refcount on TG's clamp changes Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 13/16] sched/core: uclamp: use percentage clamp values Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 14/16] sched/core: uclamp: request CAP_SYS_ADMIN by default Patrick Bellasi
2018-09-04 13:47   ` Juri Lelli
2018-09-06 14:40     ` Patrick Bellasi
2018-09-06 14:59       ` Juri Lelli
2018-09-06 17:21         ` Patrick Bellasi
2018-09-14 11:10       ` Peter Zijlstra
2018-09-14 14:07         ` Patrick Bellasi
2018-09-14 14:28           ` Peter Zijlstra
2018-09-17 12:27             ` Patrick Bellasi
2018-09-21  9:13               ` Peter Zijlstra
2018-09-24 15:14                 ` Patrick Bellasi
2018-09-24 15:56                   ` Peter Zijlstra
2018-09-24 17:23                     ` Patrick Bellasi
2018-09-24 16:26                   ` Peter Zijlstra
2018-09-24 17:19                     ` Patrick Bellasi
2018-09-25 15:49                   ` Peter Zijlstra
2018-09-26 10:43                     ` Patrick Bellasi
2018-09-27 10:00                     ` Quentin Perret
2018-09-26 17:51                 ` Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 15/16] sched/core: uclamp: add clamp group discretization support Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 16/16] sched/cpufreq: uclamp: add utilization clamping for RT tasks Patrick Bellasi

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