From: Sai Gurrappadi <sgurrappadi@nvidia.com>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>,
Linux PM <linux-pm@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.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>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Boonstoppel <pboonstoppel@nvidia.com>
Subject: Re: [RFC][PATCH v3 2/2] cpufreq: schedutil: Avoid reducing frequency of busy CPUs prematurely
Date: Fri, 24 Mar 2017 12:08:24 -0700 [thread overview]
Message-ID: <58D56EA8.5050708@nvidia.com> (raw)
In-Reply-To: <CAJZ5v0jTi6BkKiCXp9PLMAoHmC_rz9bobU5nZKo-OXq2dUKSeQ@mail.gmail.com>
On 03/23/2017 06:39 PM, Rafael J. Wysocki wrote:
> On Thu, Mar 23, 2017 at 8:26 PM, Sai Gurrappadi <sgurrappadi@nvidia.com> wrote:
>> Hi Rafael,
>
> Hi,
>
>> On 03/21/2017 04:08 PM, Rafael J. Wysocki wrote:
>>> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>
>> <snip>
>>
>>>
>>> That has been attributed to CPU utilization metric updates on task
>>> migration that cause the total utilization value for the CPU to be
>>> reduced by the utilization of the migrated task. If that happens,
>>> the schedutil governor may see a CPU utilization reduction and will
>>> attempt to reduce the CPU frequency accordingly right away. That
>>> may be premature, though, for example if the system is generally
>>> busy and there are other runnable tasks waiting to be run on that
>>> CPU already.
>>>
>>> This is unlikely to be an issue on systems where cpufreq policies are
>>> shared between multiple CPUs, because in those cases the policy
>>> utilization is computed as the maximum of the CPU utilization values
>>> over the whole policy and if that turns out to be low, reducing the
>>> frequency for the policy most likely is a good idea anyway. On
>>
>> I have observed this issue even in the shared policy case (one clock domain for many CPUs). On migrate, the actual load update is split into two updates:
>>
>> 1. Add to removed_load on src_cpu (cpu_util(src_cpu) not updated yet)
>> 2. Do wakeup on dst_cpu, add load to dst_cpu
>>
>> Now if src_cpu manages to do a PELT update before 2. happens, ex: say a small periodic task woke up on src_cpu, it'll end up subtracting the removed_load from its utilization and issue a frequency update before 2. happens.
>>
>> This causes a premature dip in frequency which doesn't get corrected until the next util update that fires after rate_limit_us. The dst_cpu freq. update from step 2. above gets rate limited in this scenario.
>
> Interesting, and this seems to be related to last_freq_update_time
> being per-policy (which it has to be, because frequency updates are
> per-policy too and that's what we need to rate-limit).
>
Correct.
> Does this happen often enough to be a real concern in practice on
> those configurations, though?
>
> The other CPUs in the policy need to be either idle (so schedutil
> doesn't take them into account at all) or lightly utilized for that to
> happen, so that would affect workloads with one CPU hog type of task
> that is migrated from one CPU to another within a policy and that
> doesn't happen too often AFAICS.
So it is possible, even likely in some cases for a heavy CPU task to migrate on wakeup between the policy->cpus via select_idle_sibling() if the prev_cpu it was on was !idle on wakeup.
This style of heavy thread + lots of light work is a common pattern on Android (games, browsing, etc.) given how Android does its threading for ipc (Binder stuff) + its rendering/audio pipelines.
I unfortunately don't have any numbers atm though.
-Sai
next prev parent reply other threads:[~2017-03-24 19:10 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
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 [this message]
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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