From: "Rafael J. Wysocki" <rjw@rjwysocki.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linux PM <linux-pm@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
Viresh Kumar <viresh.kumar@linaro.org>,
Juri Lelli <juri.lelli@arm.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Patrick Bellasi <patrick.bellasi@arm.com>,
Joel Fernandes <joelaf@google.com>,
Morten Rasmussen <morten.rasmussen@arm.com>,
Ingo Molnar <mingo@redhat.com>
Subject: Re: [RFC][PATCH 2/2] cpufreq: schedutil: Force max frequency on busy CPUs
Date: Mon, 20 Mar 2017 14:04:16 +0100 [thread overview]
Message-ID: <3708982.k2nGuG987y@aspire.rjw.lan> (raw)
In-Reply-To: <20170320125009.nmi3mvrxappjrvgo@hirez.programming.kicks-ass.net>
On Monday, March 20, 2017 01:50:09 PM Peter Zijlstra wrote:
> On Mon, Mar 20, 2017 at 01:35:12PM +0100, Rafael J. Wysocki wrote:
> > On Monday, March 20, 2017 11:36:45 AM Peter Zijlstra wrote:
> > > On Sun, Mar 19, 2017 at 02:34:32PM +0100, Rafael J. Wysocki wrote:
> > > > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > > >
> > > > The PELT metric used by the schedutil governor underestimates the
> > > > CPU utilization in some cases. The reason for that may be time spent
> > > > in interrupt handlers and similar which is not accounted for by PELT.
> > > >
> > > > That can be easily demonstrated by running kernel compilation on
> > > > a Sandy Bridge Intel processor, running turbostat in parallel with
> > > > it and looking at the values written to the MSR_IA32_PERF_CTL
> > > > register. Namely, the expected result would be that when all CPUs
> > > > were 100% busy, all of them would be requested to run in the maximum
> > > > P-state, but observation shows that this clearly isn't the case.
> > > > The CPUs run in the maximum P-state for a while and then are
> > > > requested to run slower and go back to the maximum P-state after
> > > > a while again. That causes the actual frequency of the processor to
> > > > visibly oscillate below the sustainable maximum in a jittery fashion
> > > > which clearly is not desirable.
> > > >
> > > > To work around this issue use the observation that, from the
> > > > schedutil governor's perspective, CPUs that are never idle should
> > > > always run at the maximum frequency and make that happen.
> > > >
> > > > To that end, add a counter of idle calls to struct sugov_cpu and
> > > > modify cpuidle_idle_call() to increment that counter every time it
> > > > is about to put the given CPU into an idle state. Next, make the
> > > > schedutil governor look at that counter for the current CPU every
> > > > time before it is about to start heavy computations. If the counter
> > > > has not changed for over SUGOV_BUSY_THRESHOLD time (equal to 50 ms),
> > > > the CPU has not been idle for at least that long and the governor
> > > > will choose the maximum frequency for it without looking at the PELT
> > > > metric at all.
> > >
> > > Why the time limit?
> >
> > One iteration appeared to be a bit too aggressive, but honestly I think
> > I need to check again if this thing is regarded as viable at all.
> >
>
> I don't hate the idea; if we don't hit idle; we shouldn't shift down.
OK
> I just wonder if we don't already keep a idle-seqcount somewhere; NOHZ and
> RCU come to mind as things that might already use something like that.
NOHZ does that, but I did't want this to artificially depend on NOHZ. That said,
yes, we can use that one too.
next prev parent reply other threads:[~2017-03-20 13:16 UTC|newest]
Thread overview: 71+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-19 13:21 [PATCH 0/2] cpufreq: schedutil: Fix and optimization Rafael J. Wysocki
2017-03-19 13:30 ` [PATCH 1/2] cpufreq: schedutil: Fix per-CPU structure initialization in sugov_start() Rafael J. Wysocki
2017-03-20 3:28 ` Viresh Kumar
2017-03-20 12:36 ` Rafael J. Wysocki
2017-03-19 13:34 ` [RFC][PATCH 2/2] cpufreq: schedutil: Force max frequency on busy CPUs Rafael J. Wysocki
2017-03-19 21:24 ` Rafael J. Wysocki
2017-03-19 21:42 ` Rafael J. Wysocki
2017-03-20 10:38 ` Peter Zijlstra
2017-03-20 12:31 ` Rafael J. Wysocki
2017-03-20 3:57 ` Viresh Kumar
2017-03-20 8:26 ` Vincent Guittot
2017-03-20 12:34 ` Patrick Bellasi
2017-03-22 23:56 ` Joel Fernandes
2017-03-23 22:08 ` Vincent Guittot
2017-03-25 3:48 ` Joel Fernandes
2017-03-27 6:59 ` Vincent Guittot
2017-03-20 12:59 ` Rafael J. Wysocki
2017-03-20 13:20 ` Vincent Guittot
2017-03-20 12:48 ` Rafael J. Wysocki
2017-03-20 10:36 ` Peter Zijlstra
2017-03-20 12:35 ` Rafael J. Wysocki
2017-03-20 12:50 ` Peter Zijlstra
2017-03-20 13:04 ` Rafael J. Wysocki [this message]
2017-03-20 13:06 ` Patrick Bellasi
2017-03-20 13:05 ` Rafael J. Wysocki
2017-03-20 14:13 ` Patrick Bellasi
2017-03-20 21:46 ` [RFC][PATCH v2 2/2] cpufreq: schedutil: Avoid decreasing frequency of " Rafael J. Wysocki
2017-03-21 6:40 ` Viresh Kumar
2017-03-21 12:30 ` Rafael J. Wysocki
2017-03-21 8:50 ` Vincent Guittot
2017-03-21 11:56 ` Patrick Bellasi
2017-03-21 13:22 ` Peter Zijlstra
2017-03-21 13:37 ` Vincent Guittot
2017-03-21 14:03 ` Peter Zijlstra
2017-03-21 14:18 ` Vincent Guittot
2017-03-21 14:25 ` Patrick Bellasi
[not found] ` <CAKfTPtALorn7HNpz4LOfWWSc3u+9y5iHB5byzfTHGQXDA+tVJQ@mail.gmail.com>
2017-03-21 14:58 ` Peter Zijlstra
2017-03-21 17:00 ` Vincent Guittot
2017-03-21 17:01 ` Vincent Guittot
2017-03-21 14:26 ` Rafael J. Wysocki
2017-03-21 14:38 ` Patrick Bellasi
2017-03-21 14:46 ` Rafael J. Wysocki
2017-03-21 14:50 ` Rafael J. Wysocki
2017-03-21 15:04 ` Peter Zijlstra
2017-03-21 15:18 ` Rafael J. Wysocki
2017-03-21 17:00 ` Peter Zijlstra
2017-03-21 17:17 ` Rafael J. Wysocki
2017-03-21 15:08 ` Patrick Bellasi
2017-03-21 15:18 ` Peter Zijlstra
2017-03-21 19:28 ` Patrick Bellasi
2017-03-21 15:02 ` Peter Zijlstra
2017-03-21 11:50 ` Patrick Bellasi
2017-03-21 23:08 ` [RFC][PATCH v3 2/2] cpufreq: schedutil: Avoid reducing frequency of busy CPUs prematurely Rafael J. Wysocki
2017-03-22 9:26 ` Peter Zijlstra
2017-03-22 9:54 ` Viresh Kumar
2017-03-23 1:04 ` Joel Fernandes
2017-03-23 19:26 ` Sai Gurrappadi
2017-03-23 20:48 ` Sai Gurrappadi
2017-03-24 1:39 ` Rafael J. Wysocki
2017-03-24 19:08 ` Sai Gurrappadi
2017-03-25 1:14 ` Sai Gurrappadi
2017-03-25 1:39 ` Rafael J. Wysocki
2017-03-27 7:04 ` Vincent Guittot
2017-03-27 21:01 ` Sai Gurrappadi
2017-03-27 21:11 ` Rafael J. Wysocki
2017-05-08 3:49 ` Wanpeng Li
2017-05-08 4:01 ` Viresh Kumar
2017-05-08 5:15 ` Wanpeng Li
2017-05-08 22:16 ` Rafael J. Wysocki
2017-05-08 22:36 ` Wanpeng Li
2017-05-08 23:01 ` Rafael J. Wysocki
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