Re: [PATCH] cpufreq: conservative: Ignore idle periods when a policy CPU is busy

Zhongqiu wrote:

Hi Shengming,
Thanks for the patch.

Hi Zhongqiu,
Thanks for the review!

On 9/2/2026 3:47 PM, hu.shengming@zte.com.cn wrote:

quoted

From: Shengming Hu <redacted>

For a shared cpufreq policy, dbs_update() derives the load from the
highest utilization among its CPUs, but it also records deferred idle
periods from any CPU whose idle time exceeds two sampling intervals.

This lets a single update report both a high load (from a busy CPU)
and several deferred idle periods (from an idle sibling). Since
conservative applies the deferred down steps before the up step
triggered by the high load, the down steps can outweigh the single
up step.

The issue reproduces on a policy shared by CPUs 2 and 3: a CPU-bound
SCHED_EXT task keeps CPU 2 at 100% utilization while CPU 3 stays
idle. On this system SCHED_EXT generates update-util callbacks less
frequently than CFS, so DBS updates are sparse, tracing shows:

  load=100 idle_periods=7 interval=59 ms
  load=100 idle_periods=4 interval=39 ms
  load=100 idle_periods=2 interval=19 ms
  load=100 idle_periods=7 interval=59 ms

With the default 5% step and a 2.6 GHz ceiling, conservative first
removes seven 130 MHz steps and then adds only one. Repeating this
sequence keeps the policy near 530 MHz despite CPU 2 being fully busy.

Only retain deferred idle periods when every CPU in the policy meets
the long-idle condition. This keeps the existing behavior for
single-CPU and fully idle shared policies, while preventing an idle
sibling from downscaling a policy that contains a busy CPU.

Cc: stable@vger.kernel.org
Fixes: 00bfe05889e9 ("cpufreq: conservative: Decrease frequency faster for deferred updates")
Reviewed-by: Luo Haiyang <redacted>
Reviewed-by: Run Zhang <redacted>
Signed-off-by: Shengming Hu <redacted>
---
  drivers/cpufreq/cpufreq_governor.c | 5 ++++-
  1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
index 710d93ec89b5..64eb6b5f08a4 100644
--- a/drivers/cpufreq/cpufreq_governor.c
+++ b/drivers/cpufreq/cpufreq_governor.c

@@ -126,6 +126,7 @@ unsigned int dbs_update(struct cpufreq_policy *policy)
      unsigned int ignore_nice = dbs_data->ignore_nice_load;
      unsigned int max_load = 0, idle_periods = UINT_MAX;
      unsigned int sampling_rate, io_busy, j;
+    bool all_cpus_idle = true;
      u64 cur_nice;

      /*

@@ -233,13 +234,15 @@ unsigned int dbs_update(struct cpufreq_policy *policy)

              if (periods < idle_periods)
                  idle_periods = periods;
+        } else {
+            all_cpus_idle = false;

The problem is real, but I don't think this condition is the right one.
idle_time > 2 * sampling_rate tells us how many sampling periods were
deferred for that CPU, so its negation means "this CPU was sampled on
time", not "this CPU is busy".

Since all_cpus_idle is per-policy, one such CPU is enough to discard the
deferred periods for the whole policy, and in a shared policy it is
possible. That effectively disables the optimization from 00bfe05889e9
for shared policies, which is the opposite of what we want for power.

Agreed that not meeting the long-idle condition does not necessarily
mean that the CPU was busy. The condition is based on accumulated idle
time, so it is not a reliable indication of whether that CPU should
prevent deferred downscaling.

What matters is whether the CPU was busy over the sample, that is,
whether the skipped sampling periods would have led to a frequency
reduction at all. It seems more appropriate to key that off the load
measured over the sample (kept separate from the possibly inherited one)
against up_threshold, so an idle-but-punctually-sampled sibling does not

Thanks for the suggestion. I agree that the load actually measured over
the current sample should be kept separate from the load that may inherit
prev_load. However, I don't think up_threshold is the appropriate
boundary for deciding whether deferred down steps should be applied.

For example, suppose CPU A has been idle for several sampling periods
while CPU B has a sustained load of 75%, with up_threshold at 80 and
down_threshold at 20. The policy is then in conservative's hold region,
so the load itself would trigger neither an increase nor a decrease.
If deferred downscaling is gated only by up_threshold, CPU B would
not block it, so CPU A's deferred idle periods could still reduce
the policy frequency.

I think deferred down steps should instead be applied only when the
maximum load actually measured across the policy is below
down_threshold. To keep this independent of the load returned by
dbs_update(), which may inherit prev_load, we could record the maximum
measured load separately in struct policy_dbs_info, for example as
max_sample_load.

The conservative governor could then gate the deferred reductions with
something like:

    if (policy_dbs->max_sample_load < cs_tuners->down_threshold &&
        policy_dbs->idle_periods < UINT_MAX) {
        ...
    }

This preserves deferred downscaling when the measured policy load is
below down_threshold, while avoiding deferred reductions when any CPU
is in either the hold or upscale region.

May I know could you comment and try this patch on your scenario?  Once 
everyone agrees I can send this formally:

I'll rework the patch along these lines, keeping the measured load
separate from the inherited load and using down_threshold for the
deferred-downscale condition. 

I'll send a v2, with a Suggested-by tag for your suggestion.

--
With Best Regards,
Shengming