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From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
	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: Wed, 9 May 2018 10:56:11 +0200	[thread overview]
Message-ID: <CAJZ5v0gAuhUsEg_yLw1kZYE_MnaLFR+X1c9OZCROF2u17RaKOg@mail.gmail.com> (raw)
In-Reply-To: <20180509084128.s3nu57njyep4tw2w@vireshk-i7>

On Wed, May 9, 2018 at 10:41 AM, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> On 08-05-18, 22:54, Rafael J. Wysocki wrote:
>> 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.
>
> I added the "Fixes" tag because this is exactly the commit after which
> this problem started, isn't it?
>
>> 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.
>
> I thought not overwriting next_freq makes things much simpler and easy
> to review.

I'm kind of concerned about updating the limits via sysfs in which
case the cached next frequency may be out of range, so it's better to
invalidate it right away then.

> What else do you have in mind to solve this problem ?

Something like the below?

---
 kernel/sched/cpufreq_schedutil.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Index: linux-pm/kernel/sched/cpufreq_schedutil.c
===================================================================
--- linux-pm.orig/kernel/sched/cpufreq_schedutil.c
+++ linux-pm/kernel/sched/cpufreq_schedutil.c
@@ -305,7 +305,8 @@ static void sugov_update_single(struct u
      * Do not reduce the frequency if the CPU has not been idle
      * recently, as the reduction is likely to be premature then.
      */
-    if (busy && next_f < sg_policy->next_freq) {
+    if (busy && next_f < sg_policy->next_freq &&
+        sg_policy->next_freq != UINT_MAX) {
         next_f = sg_policy->next_freq;

         /* Reset cached freq as next_freq has changed */

  reply	other threads:[~2018-05-09  8:56 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
2018-05-09  8:41   ` Viresh Kumar
2018-05-09  8:56     ` Rafael J. Wysocki [this message]
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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