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Message-ID: <Z_U_LN0AtH_n4YtE@sultan-box.localdomain>
Date: Tue, 8 Apr 2025 08:22:20 -0700
From: Sultan Alsawaf <sultan@...neltoast.com>
To: Stephan Gerhold <stephan.gerhold@...aro.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
Viresh Kumar <viresh.kumar@...aro.org>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Valentin Schneider <vschneid@...hat.com>, linux-pm@...r.kernel.org,
linux-kernel@...r.kernel.org, regressions@...ts.linux.dev,
Johan Hovold <johan@...nel.org>
Subject: Re: [PATCH 1/2] cpufreq: schedutil: Fix superfluous updates caused
by need_freq_update
Hi Stephan,
On Tue, Apr 08, 2025 at 10:59:31AM +0200, Stephan Gerhold wrote:
> Hi,
>
> On Wed, Dec 11, 2024 at 05:57:32PM -0800, Sultan Alsawaf wrote:
> > From: "Sultan Alsawaf (unemployed)" <sultan@...neltoast.com>
> >
> > A redundant frequency update is only truly needed when there is a policy
> > limits change with a driver that specifies CPUFREQ_NEED_UPDATE_LIMITS.
> >
> > In spite of that, drivers specifying CPUFREQ_NEED_UPDATE_LIMITS receive a
> > frequency update _all the time_, not just for a policy limits change,
> > because need_freq_update is never cleared.
> >
> > Furthermore, ignore_dl_rate_limit()'s usage of need_freq_update also leads
> > to a redundant frequency update, regardless of whether or not the driver
> > specifies CPUFREQ_NEED_UPDATE_LIMITS, when the next chosen frequency is the
> > same as the current one.
> >
> > Fix the superfluous updates by only honoring CPUFREQ_NEED_UPDATE_LIMITS
> > when there's a policy limits change, and clearing need_freq_update when a
> > requisite redundant update occurs.
> >
> > This is neatly achieved by moving up the CPUFREQ_NEED_UPDATE_LIMITS test
> > and instead setting need_freq_update to false in sugov_update_next_freq().
> >
> > Signed-off-by: Sultan Alsawaf (unemployed) <sultan@...neltoast.com>
> > ---
> > kernel/sched/cpufreq_schedutil.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 28c77904ea74..e51d5ce730be 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -83,7 +83,7 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
> >
> > if (unlikely(sg_policy->limits_changed)) {
> > sg_policy->limits_changed = false;
> > - sg_policy->need_freq_update = true;
> > + sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
> > return true;
> > }
> >
> > @@ -96,7 +96,7 @@ static bool sugov_update_next_freq(struct sugov_policy *sg_policy, u64 time,
> > unsigned int next_freq)
> > {
> > if (sg_policy->need_freq_update)
> > - sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
> > + sg_policy->need_freq_update = false;
> > else if (sg_policy->next_freq == next_freq)
> > return false;
> >
>
> This patch breaks cpufreq throttling (e.g. for thermal cooling) for
> cpufreq drivers that:
>
> - Have policy->fast_switch_enabled/fast_switch_possible set, but
> - Do not have CPUFREQ_NEED_UPDATE_LIMITS flag set
>
> There are several examples for this in the tree (search for
> "fast_switch_possible"). Of all those drivers, only intel-pstate and
> amd-pstate (sometimes) set CPUFREQ_NEED_UPDATE_LIMITS.
>
> I can reliably reproduce this with scmi-cpufreq on a Qualcomm X1E
> laptop:
>
> 1. I added some low temperature trip points in the device tree,
> together with passive cpufreq cooling.
> 2. I run a CPU stress test on all CPUs and monitor the temperatures
> and CPU frequencies.
>
> When using "performance" governor instead of "schedutil", the CPU
> frequencies are being throttled as expected, as soon as the temperature
> trip points are reached.
>
> When using "schedutil", the CPU frequencies stay at maximum as long as
> the stress test is running. No throttling happens, so the device heats
> up far beyond the defined temperature trip points. Throttling is applied
> only after stopping the stress test, since this forces schedutil to
> re-evaluate the CPU frequency.
>
> Reverting this commit fixes the problem.
>
> Looking at the code, I think the problem is that:
> - sg_policy->limits_changed does not result in
> sg->policy->need_freq_update without CPUFREQ_NEED_UPDATE_LIMITS
> anymore, and
> - Without sg->policy->need_freq_update, get_next_freq() skips calling
> cpufreq_driver_resolve_freq(), which would normally apply the policy
> min/max constraints.
>
> Do we need to set CPUFREQ_NEED_UPDATE_LIMITS for all cpufreq drivers
> that set policy->fast_switch_possible? If I'm reading the documentation
> comment correctly, that flag is just supposed to enable notifications if
> the policy min/max changes, but the resolved target frequency is still
> the same. This is not the case here, the target frequency needs to be
> throttled, but schedutil isn't applying the new limits.
>
> Any suggestions how to fix this? I'm happy to test patches with my
> setup.
Thank you for reporting this. As I see it, sg_policy->need_freq_update is
working correctly now; however, sg_policy->limits_changed relied on the broken
behavior of sg_policy->need_freq_update and therefore sg_policy->limits_changed
needs to be fixed.
Can you try this patch:
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 1a19d69b91ed3..f37b999854d52 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -82,7 +82,6 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
return false;
if (unlikely(sg_policy->limits_changed)) {
- sg_policy->limits_changed = false;
sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
return true;
}
@@ -171,9 +170,11 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
freq = get_capacity_ref_freq(policy);
freq = map_util_freq(util, freq, max);
- if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)
+ if (freq == sg_policy->cached_raw_freq && !sg_policy->limits_changed &&
+ !sg_policy->need_freq_update)
return sg_policy->next_freq;
+ sg_policy->limits_changed = false;
sg_policy->cached_raw_freq = freq;
return cpufreq_driver_resolve_freq(policy, freq);
}
>
> Thanks,
> Stephan
>
> #regzbot introduced: 8e461a1cb43d69d2fc8a97e61916dce571e6bb31
Thanks,
Sultan
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