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From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Rafael Wysocki <rjw@rjwysocki.net>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Linux PM <linux-pm@vger.kernel.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Claudio Scordino <claudio@evidence.eu.com>,
	Patrick Bellasi <patrick.bellasi@arm.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Joel Fernandes <joelaf@google.com>,
	"4 . 12+" <stable@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] sched/schedutil: Don't set next_freq to UINT_MAX
Date: Tue, 8 May 2018 22:54:32 +0200	[thread overview]
Message-ID: <CAJZ5v0gVpOT81WPFc8shkPa0vzteb6s2r9dBYW9VGz+wcuffGw@mail.gmail.com> (raw)
In-Reply-To: <872c3f8690d9362820639d91a807e535f10a9a36.1525761635.git.viresh.kumar@linaro.org>

On Tue, May 8, 2018 at 8:42 AM, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> The schedutil driver sets sg_policy->next_freq to UINT_MAX on certain
> occasions:
> - In sugov_start(), when the schedutil governor is started for a group
>   of CPUs.
> - And whenever we need to force a freq update before rate-limit
>   duration, which happens when:
>   - there is an update in cpufreq policy limits.
>   - Or when the utilization of DL scheduling class increases.
>
> In return, get_next_freq() doesn't return a cached next_freq value but
> instead recalculates the next frequency. This has some side effects
> though and may significantly delay a required increase in frequency.
>
> In sugov_update_single() we try to avoid decreasing frequency if the CPU
> has not been idle recently. Consider this scenario, the available range
> of frequencies for a CPU are from 800 MHz to 2.5 GHz and current
> frequency is 800 MHz. From one of the call paths
> sg_policy->need_freq_update is set to true and hence
> sg_policy->next_freq is set to UINT_MAX. Now if the CPU had been busy,
> next_f will always be less than UINT_MAX, whatever the value of next_f
> is. And so even when we wanted to increase the frequency, we will
> overwrite next_f with UINT_MAX and will not change the frequency
> eventually. This will continue until the time CPU stays busy. This isn't
> cross checked with any specific test cases, but rather based on general
> code review.
>
> Fix that by not resetting the sg_policy->need_freq_update flag from
> sugov_should_update_freq() but get_next_freq() and we wouldn't need to
> overwrite sg_policy->next_freq anymore.
>
> Cc: 4.12+ <stable@vger.kernel.org> # 4.12+
> Fixes: b7eaf1aab9f8 ("cpufreq: schedutil: Avoid reducing frequency of busy CPUs prematurely")

The rest of the chantelog is totally disconnected from this commit.

So the problem is that sugov_update_single() doesn't check the special
UNIT_MAX value before assigning sg_policy->next_freq to next_f.  Fair
enough.

I don't see why the patch is the right fix for that, however.

Thanks,
Rafael

  reply	other threads:[~2018-05-08 20:54 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-08  6:42 Viresh Kumar
2018-05-08 20:54 ` Rafael J. Wysocki [this message]
2018-05-09  8:41   ` Viresh Kumar
2018-05-09  8:56     ` Rafael J. Wysocki
2018-05-09  9:15       ` Viresh Kumar
2018-05-09  9:23         ` Rafael J. Wysocki
2018-05-09  9:30           ` Viresh Kumar
2018-05-09  9:32             ` Rafael J. Wysocki
2018-05-09  9:44 ` [PATCH] cpufreq: schedutil: Avoid using invalid next_freq Rafael J. Wysocki
2018-05-09  9:46   ` Viresh Kumar
2018-05-09 10:35   ` [PATCH V2] sched/schedutil: Don't set next_freq to UINT_MAX Viresh Kumar
2018-05-11 20:47     ` [V2] " Joel Fernandes
2018-05-17 10:33       ` Rafael J. Wysocki

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