From: "Rafael J. Wysocki" <rjw@rjwysocki.net>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>,
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>,
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>
Subject: Re: [RFC][PATCH v3 2/2] cpufreq: schedutil: Avoid reducing frequency of busy CPUs prematurely
Date: Tue, 09 May 2017 01:01:43 +0200 [thread overview]
Message-ID: <1827227.Nyzl5ssJXb@aspire.rjw.lan> (raw)
In-Reply-To: <CANRm+Cz+to-hLcmMXNYPf8yeZkg6Fa8AvW-j4C80HR4ewQ085Q@mail.gmail.com>
On Tuesday, May 09, 2017 06:36:14 AM Wanpeng Li wrote:
> 2017-05-09 6:16 GMT+08:00 Rafael J. Wysocki <rjw@rjwysocki.net>:
> > On Monday, May 08, 2017 09:31:19 AM Viresh Kumar wrote:
> >> On 08-05-17, 11:49, Wanpeng Li wrote:
> >> > Hi Rafael,
> >> > 2017-03-22 7:08 GMT+08:00 Rafael J. Wysocki <rjw@rjwysocki.net>:
> >> > > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> >> > >
> >> > > The way the schedutil governor uses the PELT metric causes it to
> >> > > underestimate the CPU utilization in some cases.
> >> > >
> >> > > 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.
> >> > >
> >> > > 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
> >> >
> >> > Sorry for one question maybe not associated with this patch. If the
> >> > cpufreq policy is shared between multiple CPUs, the function
> >> > intel_cpufreq_target() just updates IA32_PERF_CTL MSR of the cpu
> >> > which is managing this policy, I wonder whether other cpus which are
> >> > affected should also update their per-logical cpu's IA32_PERF_CTL MSR?
> >>
> >> The CPUs share the policy when they share their freq/voltage rails and so
> >> changing perf state of one CPU should result in that changing for all the CPUs
> >> in that policy. Otherwise, they can't be considered to be part of the same
> >> policy.
> >
> > To be entirely precise, this depends on the granularity of the HW interface.
> >
> > If the interface is per-logical-CPU, we will use it this way for efficiency
> > reasons and even if there is some coordination on the HW side, the information
> > on how exactly it works usually is limited.
>
> I check it on several Xeon servers on hand, however, I didn't find
> /sys/devices/system/cpu/cpufreq/policyx/affected_cpus can affect more
> than one logical cpu, so I guess most of Xeon servers are not support
> shared cpufreq policy, then which kind of boxes support that?
On Intel the interface for performance scaling is per-logical-CPU in general.
Thanks,
Rafael
prev parent reply other threads:[~2017-05-08 23:08 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
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 [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=1827227.Nyzl5ssJXb@aspire.rjw.lan \
--to=rjw@rjwysocki.net \
--cc=joelaf@google.com \
--cc=juri.lelli@arm.com \
--cc=kernellwp@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-pm@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=morten.rasmussen@arm.com \
--cc=patrick.bellasi@arm.com \
--cc=peterz@infradead.org \
--cc=srinivas.pandruvada@linux.intel.com \
--cc=tglx@linutronix.de \
--cc=vincent.guittot@linaro.org \
--cc=viresh.kumar@linaro.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).