From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Doug Smythies <dsmythies@telus.net>,
"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>,
Joel Fernandes <joel@joelfernandes.org>,
"v4 . 18+" <stable@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] cpufreq: schedutil: Don't skip freq update when limits change
Date: Thu, 1 Aug 2019 09:47:28 +0200 [thread overview]
Message-ID: <CAJZ5v0h7GPT3Z_oWz=WfJon=wg3bgS3KVMOATEYvdTM2ywuHOA@mail.gmail.com> (raw)
In-Reply-To: <20190801061700.dl33rtilvg44obzu@vireshk-i7>
On Thu, Aug 1, 2019 at 8:17 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
>
> On 31-07-19, 17:20, Doug Smythies wrote:
> > Hi Viresh,
> >
> > Summary:
> >
> > The old way, using UINT_MAX had two purposes: first,
> > as a "need to do a frequency update" flag; but also second, to
> > force any subsequent old/new frequency comparison to NOT be "the same,
> > so why bother actually updating" (see: sugov_update_next_freq). All
> > patches so far have been dealing with the flag, but only partially
> > the comparisons. In a busy system, and when schedutil.c doesn't actually
> > know the currently set system limits, the new frequency is dominated by
> > values the same as the old frequency. So, when sugov_fast_switch calls
> > sugov_update_next_freq, false is usually returned.
>
> And finally we know "Why" :)
>
> Good work Doug. Thanks for taking it to the end.
>
> > However, if we move the resetting of the flag and add another condition
> > to the "no need to actually update" decision, then perhaps this patch
> > version 1 will be O.K. It seems to be. (see way later in this e-mail).
>
> > With all this new knowledge, how about going back to
> > version 1 of this patch, and then adding this:
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 808d32b..f9156db 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -100,7 +100,12 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
> > static bool sugov_update_next_freq(struct sugov_policy *sg_policy, u64 time,
> > unsigned int next_freq)
> > {
> > - if (sg_policy->next_freq == next_freq)
> > + /*
> > + * Always force an update if the flag is set, regardless.
> > + * In some implementations (intel_cpufreq) the frequency is clamped
> > + * further downstream, and might not actually be different here.
> > + */
> > + if (sg_policy->next_freq == next_freq && !sg_policy->need_freq_update)
> > return false;
>
> This is not correct because this is an optimization we have in place
> to make things more efficient. And it was working by luck earlier and
> my patch broke it for good :)
OK, so since we know why it was wrong now, why don't we just revert
it? Plus maybe add some comment explaining the rationale in there?
next prev parent reply other threads:[~2019-08-01 7:47 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-18 6:26 [PATCH] Revert "cpufreq: schedutil: Don't set next_freq to UINT_MAX" Doug Smythies
2019-07-18 10:28 ` Viresh Kumar
2019-07-18 15:46 ` Doug Smythies
2019-07-22 6:49 ` Viresh Kumar
2019-07-22 6:51 ` [PATCH] cpufreq: schedutil: Don't skip freq update when limits change Viresh Kumar
2019-07-23 7:10 ` Doug Smythies
2019-07-23 9:13 ` Rafael J. Wysocki
2019-07-23 9:15 ` Viresh Kumar
2019-07-23 10:27 ` Rafael J. Wysocki
2019-07-24 11:43 ` Viresh Kumar
2019-07-25 15:20 ` Doug Smythies
2019-07-26 3:26 ` Viresh Kumar
2019-07-26 6:57 ` Viresh Kumar
2019-07-29 7:55 ` Doug Smythies
2019-07-29 8:32 ` Viresh Kumar
2019-07-29 8:37 ` Rafael J. Wysocki
2019-08-01 0:20 ` Doug Smythies
2019-08-01 6:17 ` Viresh Kumar
2019-08-01 7:47 ` Rafael J. Wysocki [this message]
2019-08-01 7:55 ` Viresh Kumar
2019-08-01 17:57 ` Doug Smythies
2019-08-02 3:48 ` Viresh Kumar
2019-08-02 9:11 ` Rafael J. Wysocki
2019-08-02 9:19 ` Rafael J. Wysocki
2019-08-06 4:00 ` Viresh Kumar
2019-07-31 2:58 Viresh Kumar
2019-07-31 23:19 ` Doug Smythies
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