From: Ionela Voinescu <ionela.voinescu@arm.com>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
linux-pm@vger.kernel.org,
Catalin Marinas <catalin.marinas@arm.com>,
Rafael Wysocki <rjw@rjwysocki.net>,
linux-kernel@vger.kernel.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Sudeep Holla <sudeep.holla@arm.com>,
Will Deacon <will@kernel.org>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH V3 1/2] topology: Allow multiple entities to provide sched_freq_tick() callback
Date: Wed, 17 Feb 2021 11:57:26 +0000 [thread overview]
Message-ID: <20210217115726.GA25441@arm.com> (raw)
In-Reply-To: <20210217114027.ashqh67hrfk4hwib@vireshk-i7>
On Wednesday 17 Feb 2021 at 17:10:27 (+0530), Viresh Kumar wrote:
> On 17-02-21, 11:30, Ionela Voinescu wrote:
> > The problem is not topology_scale_freq_invariant() but whether a scale
> > factor is set for some CPUs.
> >
> > Scenario (test system above):
> > - "AMUs" are only supported for [1-2],
> > - cpufreq_supports_freq_invariance() -> false
> >
> > What should happen:
> > - topology_scale_freq_invariant() -> false (passed)
> > - all CPUs should have their freq_scale unmodified (1024) - (failed)
> > because only 2 out of 6 CPUs have a method of setting a scale factor
> >
> > What does happen:
> > - arch_set_freq_tick() -> topology_set_freq_tick() will set a scale
> > factor for [1-2] based on AMUs. This should not happen. We will end
> > up with invariant signals for bigs and signals that are not freq
> > invariant for littles.
>
> Another case. cpufreq is included as a module and AMU is implemented
> partially.
>
> - first time cpufreq driver is inserted, we set up everything and
> freq_scale gets updated on ticks.
>
> - remove cpufreq driver, we are back in same situation.
>
Yes, but the littles (lacking AMUs) would have had a scale factor set
through arch_set_freq_scale() which will correspond to the last
frequency change through the cpufreq driver. When removing the driver,
it's unlikely that the frequency of littles will change (no driver).
- topology_scale_freq_invariant() will still return true.
- littles would still have a scale factor set which is likely accurate
- bigs will continue updating the scale factor through AMUs.
See a very useful comment someone added recently :) :
"""
+ /*
+ * We don't need to handle CPUFREQ_REMOVE_POLICY event as the AMU
+ * counters don't have any dependency on cpufreq driver once we have
+ * initialized AMU support and enabled invariance. The AMU counters will
+ * keep on working just fine in the absence of the cpufreq driver, and
+ * for the CPUs for which there are no counters available, the last set
+ * value of freq_scale will remain valid as that is the frequency those
+ * CPUs are running at.
+ */
"""
> We can't control it that way.. Or we add another call layer in middle
> before the tick-handler gets called for AMU, which will check if we
> are fully invariant or not ?
>
I would avoid additional work done on the tick, especially for a scenario which
is unlikely. If you think this case is worth supporting, it might be best to do
it at CPUFREQ_REMOVE_POLICY event.
Thanks,
Ionela.
> --
> viresh
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next prev parent reply other threads:[~2021-02-17 11:58 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-28 10:48 [PATCH V3 0/2] cpufreq: cppc: Add support for frequency invariance Viresh Kumar
2021-01-28 10:48 ` [PATCH V3 1/2] topology: Allow multiple entities to provide sched_freq_tick() callback Viresh Kumar
2021-02-03 11:45 ` Ionela Voinescu
2021-02-05 9:14 ` Viresh Kumar
2021-02-17 0:24 ` Ionela Voinescu
2021-02-17 4:25 ` Viresh Kumar
2021-02-17 11:30 ` Ionela Voinescu
2021-02-17 11:40 ` Viresh Kumar
2021-02-17 11:57 ` Ionela Voinescu [this message]
2021-02-18 7:23 ` Viresh Kumar
2021-02-18 9:33 ` Viresh Kumar
2021-02-18 16:36 ` Ionela Voinescu
2021-02-19 4:58 ` Viresh Kumar
2021-02-19 9:44 ` Ionela Voinescu
2021-02-19 9:48 ` Viresh Kumar
2021-01-28 10:48 ` [PATCH V3 2/2] cpufreq: cppc: Add support for frequency invariance Viresh Kumar
2021-02-18 16:35 ` Ionela Voinescu
2021-02-22 11:00 ` Ionela Voinescu
2021-02-22 11:04 ` Viresh Kumar
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