Re: [PATCH 1/2] cpufreq: schedutil: Fix superfluous updates caused by need_freq_update

[Sultan Alsawaf <sultan@kerneltoast.com>]

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@kerneltoast.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@kerneltoast.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