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From: Quentin Perret <quentin.perret@arm.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	"open list:THERMAL" <linux-pm@vger.kernel.org>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	Ingo Molnar <mingo@redhat.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Chris Redpath <chris.redpath@arm.com>,
	Patrick Bellasi <patrick.bellasi@arm.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Thara Gopinath <thara.gopinath@linaro.org>,
	viresh kumar <viresh.kumar@linaro.org>,
	Todd Kjos <tkjos@google.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	"Cc: Steve Muckle" <smuckle@google.com>,
	adharmap@quicinc.com, "Kannan, Saravana" <skannan@quicinc.com>,
	pkondeti@codeaurora.org, Juri Lelli <juri.lelli@redhat.com>,
	Eduardo Valentin <edubezval@gmail.com>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	currojerez@riseup.net, Javi Merino <javi.merino@kernel.org>
Subject: Re: [PATCH v5 09/14] sched: Add over-utilization/tipping point indicator
Date: Thu, 2 Aug 2018 15:14:24 +0100
Message-ID: <20180802141424.ju4jxxbk6pxw3kyq@queper01-lin> (raw)
In-Reply-To: <CAKfTPtDcoySXK0fBkDNy4wp1vsRxmiuAGT3CDZBh6Vnwyep2BA@mail.gmail.com>

On Thursday 02 Aug 2018 at 15:48:01 (+0200), Vincent Guittot wrote:
> On Thu, 2 Aug 2018 at 15:19, Quentin Perret <quentin.perret@arm.com> wrote:
> >
> > On Thursday 02 Aug 2018 at 15:08:01 (+0200), Peter Zijlstra wrote:
> > > On Thu, Aug 02, 2018 at 02:03:38PM +0100, Quentin Perret wrote:
> > > > On Thursday 02 Aug 2018 at 14:26:29 (+0200), Peter Zijlstra wrote:
> > > > > On Tue, Jul 24, 2018 at 01:25:16PM +0100, Quentin Perret wrote:
> > > > > > @@ -5100,8 +5118,17 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > > > > >                 update_cfs_group(se);
> > > > > >         }
> > > > > >
> > > > > > -       if (!se)
> > > > > > +       if (!se) {
> > > > > >                 add_nr_running(rq, 1);
> > > > > > +               /*
> > > > > > +                * The utilization of a new task is 'wrong' so wait for it
> > > > > > +                * to build some utilization history before trying to detect
> > > > > > +                * the overutilized flag.
> > > > > > +                */
> > > > > > +               if (flags & ENQUEUE_WAKEUP)
> > > > > > +                       update_overutilized_status(rq);
> > > > > > +
> > > > > > +       }
> > > > > >
> > > > > >         hrtick_update(rq);
> > > > > >  }
> > > > >
> > > > > That is a somewhat dodgy hack. There is no guarantee what so ever that
> > > > > when the task wakes next its history is any better. The comment doesn't
> > > > > reflect this I feel.
> > > >
> > > > AFAICT the main use-case here is to avoid re-enabling the load balance
> > > > and ruining all the task placement because of a tiny task. I don't
> > > > really see how we can do that differently ...
> > >
> > > Sure I realize that.. but it doesn't completely avoid it. Suppose this
> > > new task instantly blocks and wakes up again. Then its util signal will
> > > be exactly what you didn't want but we'll account it and cause the above
> > > scenario you wanted to avoid.
> >
> > That is true. ... I also realize now that this patch was written long
> > before util_est, and that also has an impact here, especially in the
> > scenario you described where the task blocks. So any wake-up after the
> > first enqueue will risk to overutilize the system, even if the task
> > blocked for ages.
> >
> > Hmm ...
> 
> Does a init value set to 0 for util_avg for newly created task can
> help in EAS in this case ?
> Current initial value is computed to prevent packing newly created
> tasks on same CPUs because it hurts performance of some benches. In
> fact it somehow assumes that newly created task will use significant
> part of the remaining capacity of a CPU and want to spread tasks. In
> EAS case, it seems that it prefer to assume that  newly created task
> are small and we can pack them and wait a bit to make sure the new
> task will be a big task and will overload the CPU

Good point, setting the util_avg to 0 for new tasks should help
filtering out those tiny tasks too. And that would match with the idea
of letting tasks build their history before looking at their util_avg ...

But there is one difference w.r.t frequency selection. The current code
won't mark the system overutilized, but will let sugov raise the
frequency when a new task is enqueued. So in case of a fork bomb, we
sort of fallback on the existing mainline strategy for both task
placement (because forkees don't go in find_energy_efficient_cpu) and
frequency selection. And I would argue this is the right thing to do
since EAS can't really help in this case.

Thoughts ?

Thanks,
Quentin

  reply index

Thread overview: 72+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-24 12:25 [PATCH v5 00/14] Energy Aware Scheduling Quentin Perret
2018-07-24 12:25 ` [PATCH v5 01/14] sched: Relocate arch_scale_cpu_capacity Quentin Perret
2018-07-24 12:25 ` [PATCH v5 02/14] sched/cpufreq: Factor out utilization to frequency mapping Quentin Perret
2018-07-24 12:25 ` [PATCH v5 03/14] PM: Introduce an Energy Model management framework Quentin Perret
2018-08-09 21:52   ` Rafael J. Wysocki
2018-08-10  8:15     ` Quentin Perret
2018-08-10  8:41       ` Rafael J. Wysocki
2018-08-10  9:12         ` Quentin Perret
2018-08-10 11:13           ` Rafael J. Wysocki
2018-08-10 12:30             ` Quentin Perret
2018-08-12  9:49               ` Rafael J. Wysocki
2018-07-24 12:25 ` [PATCH v5 04/14] PM / EM: Expose the Energy Model in sysfs Quentin Perret
2018-07-24 12:25 ` [PATCH v5 05/14] sched/topology: Reference the Energy Model of CPUs when available Quentin Perret
2018-07-24 12:25 ` [PATCH v5 06/14] sched/topology: Lowest energy aware balancing sched_domain level pointer Quentin Perret
2018-07-26 16:00   ` Valentin Schneider
2018-07-26 17:01     ` Quentin Perret
2018-07-24 12:25 ` [PATCH v5 07/14] sched/topology: Introduce sched_energy_present static key Quentin Perret
2018-07-24 12:25 ` [PATCH v5 08/14] sched/fair: Clean-up update_sg_lb_stats parameters Quentin Perret
2018-07-24 12:25 ` [PATCH v5 09/14] sched: Add over-utilization/tipping point indicator Quentin Perret
2018-08-02 12:26   ` Peter Zijlstra
2018-08-02 13:03     ` Quentin Perret
2018-08-02 13:08       ` Peter Zijlstra
2018-08-02 13:18         ` Quentin Perret
2018-08-02 13:48           ` Vincent Guittot
2018-08-02 14:14             ` Quentin Perret [this message]
2018-08-02 15:14               ` Vincent Guittot
2018-08-02 15:30                 ` Quentin Perret
2018-08-02 15:55                   ` Vincent Guittot
2018-08-02 16:00                     ` Quentin Perret
2018-08-02 16:07                       ` Vincent Guittot
2018-08-02 16:10                         ` Quentin Perret
2018-08-02 16:38                           ` Vincent Guittot
2018-08-02 16:59                             ` Quentin Perret
2018-08-03  7:48                               ` Vincent Guittot
2018-08-03  8:18                                 ` Quentin Perret
2018-08-03 13:49                                   ` Vincent Guittot
2018-08-03 14:21                                     ` Vincent Guittot
2018-08-03 15:55                                     ` Quentin Perret
2018-08-06  8:40                                       ` Vincent Guittot
2018-08-06  9:43                                         ` Quentin Perret
2018-08-06 10:45                                           ` Vincent Guittot
2018-08-06 11:02                                             ` Quentin Perret
2018-08-06 10:08                                         ` Dietmar Eggemann
2018-08-06 10:33                                           ` Vincent Guittot
2018-08-06 12:29                                             ` Dietmar Eggemann
2018-08-06 12:37                                               ` Vincent Guittot
2018-08-06 13:20                                                 ` Dietmar Eggemann
2018-08-09  9:30   ` Vincent Guittot
2018-08-09  9:38     ` Quentin Perret
2018-07-24 12:25 ` [PATCH v5 10/14] sched/cpufreq: Refactor the utilization aggregation method Quentin Perret
2018-07-30 19:35   ` skannan
2018-07-31  7:59     ` Quentin Perret
2018-07-31 19:31       ` skannan
2018-08-01  7:32         ` Rafael J. Wysocki
2018-08-01  8:23           ` Quentin Perret
2018-08-01  8:35             ` Rafael J. Wysocki
2018-08-01  9:23               ` Quentin Perret
2018-08-01  9:40                 ` Rafael J. Wysocki
2018-08-02 13:04                 ` Peter Zijlstra
2018-08-02 15:39                   ` Quentin Perret
2018-08-03 13:04                     ` Quentin Perret
2018-08-02 12:33     ` Peter Zijlstra
2018-08-02 12:45       ` Peter Zijlstra
2018-08-02 15:21         ` Quentin Perret
2018-08-02 17:36           ` Peter Zijlstra
2018-08-03 12:42             ` Quentin Perret
2018-07-24 12:25 ` [PATCH v5 11/14] sched/fair: Introduce an energy estimation helper function Quentin Perret
2018-07-24 12:25 ` [PATCH v5 12/14] sched/fair: Select an energy-efficient CPU on task wake-up Quentin Perret
2018-08-02 13:54   ` Peter Zijlstra
2018-08-02 16:21     ` Quentin Perret
2018-07-24 12:25 ` [PATCH v5 13/14] OPTIONAL: arch_topology: Start Energy Aware Scheduling Quentin Perret
2018-07-24 12:25 ` [PATCH v5 14/14] OPTIONAL: cpufreq: dt: Register an Energy Model Quentin Perret

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