From: "Doug Smythies" <dsmythies@telus.net>
To: "'Viresh Kumar'" <viresh.kumar@linaro.org>,
"'Rafael J. Wysocki'" <rafael@kernel.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>,
"'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, 25 Jul 2019 08:20:01 -0700 [thread overview]
Message-ID: <000c01d542fc$703ff850$50bfe8f0$@net> (raw)
In-Reply-To: <20190724114327.apmx35c7a4tv3qt5@vireshk-i7>
Hi,
I am having trouble keeping up.
Here is what I have so far:
On 2019.07.24 04:43 Viresh Kumar wrote:
> On 23-07-19, 12:27, Rafael J. Wysocki wrote:
>> On Tue, Jul 23, 2019 at 11:15 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
>>> Though there is one difference between intel_cpufreq and acpi_cpufreq,
>>> intel_cpufreq has fast_switch_possible=true and so it uses slightly
>>> different path in schedutil. I tried to look from that perspective as
>>> well but couldn't find anything wrong.
>>
>> acpi-cpufreq should use fast switching on the Doug's system too.
>
> Ah okay.
>
>>> If you still find intel_cpufreq to be broken, even with this patch,
It is.
>>> please set fast_switch_possible=false instead of true in
>>> __intel_pstate_cpu_init() and try tests again. That shall make it very
>>> much similar to acpi-cpufreq driver.
>>
>> I wonder if this helps.
It does not help.
>> Even so, we want fast switching to be used by intel_cpufreq.
>
> With both using fast switching it shouldn't make any difference.
>> Anyway, it looks like the change reverted by the Doug's patch
>> introduced a race condition that had not been present before. Namely,
>> need_freq_update is cleared in get_next_freq() when it is set _or_
>> when the new freq is different from the cached one, so in the latter
>> case if it happens to be set by sugov_limits() after evaluating
>> sugov_should_update_freq() (which returned 'true' for timing reasons),
>> that update will be lost now. [Previously the update would not be
>> lost, because the clearing of need_freq_update depended only on its
>> current value.] Where it matters is that in the "need_freq_update set"
>> case, the "premature frequency reduction avoidance" should not be
>> applied (as you noticed and hence the $subject patch).
>>
>> However, even with the $subject patch, need_freq_update may still be
>> set by sugov_limits() after the check added by it and then cleared by
>> get_next_freq(), so it doesn't really eliminate the problem.
>>
>> IMO eliminating would require invalidating next_freq this way or
>> another when need_freq_update is set in sugov_should_update_freq(),
>> which was done before commit ecd2884291261e3fddbc7651ee11a20d596bb514.
>
> Hmm, so to avoid locking in fast path we need two variable group to
> protect against this kind of issues. I still don't want to override
> next_freq with a special meaning as it can cause hidden bugs, we have
> seen that earlier.
>
> What about something like this then ?
I tried the patch ("patch2"). It did not fix the issue.
To summarize, all kernel 5.2 based, all intel_cpufreq driver and schedutil governor:
Test: Does a busy system respond to maximum CPU clock frequency reduction?
stock, unaltered: No.
revert ecd2884291261e3fddbc7651ee11a20d596bb514: Yes
viresh patch: No.
fast_switch edit: No.
viresh patch2: No.
References (and procedures used):
https://marc.info/?l=linux-pm&m=156346478429147&w=2
https://marc.info/?l=linux-kernel&m=156343125319461&w=2
... Doug
next prev parent reply other threads:[~2019-07-25 15:20 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 [this message]
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
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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