[PATCH] cpufreq: intel_pstate: Sync policy->cur to the pinned pstate

[Jing Wu]

When cpu->policy is CPUFREQ_POLICY_PERFORMANCE, intel_pstate_set_policy()
pins the CPU to a fixed pstate (max(min_pstate, max_perf_ratio)) and
programs it directly, precisely because, per the existing comment,
"NOHZ_FULL CPUs need this as the governor callback may not be invoked
on them". Two lines later it still unconditionally clobbers policy->cur
down to policy->min, discarding the pinned value it just computed and
applied.

arch_freq_get_on_cpu() falls back to cpufreq_quick_get(), i.e.
policy->cur, whenever its APERF/MPERF sample goes stale. A CPU whose
tick keeps running refreshes that sample constantly and rarely hits
the fallback, but an isolated CPU covered by nohz_full with a single
runnable task never gets another tick, so it permanently reports the
floor through this fallback - even though it is genuinely pinned to,
and running at, the frequency computed just above.

Set policy->cur to the exact pinned frequency (pstate * scaling) in
the CPUFREQ_POLICY_PERFORMANCE branch instead, and only fall back to
policy->min for the general case, where the frequency genuinely isn't
known without a fresh sample.

Fixes: d51847acb018 ("cpufreq: intel_pstate: set stale CPU frequency to minimum")
Co-developed-by: Qiliang Yuan <yuanql9@xxxxxxxxxxxxxxx>
Signed-off-by: Qiliang Yuan <yuanql9@xxxxxxxxxxxxxxx>
Signed-off-by: Jing Wu <realwujing@xxxxxxxxx>
---
 drivers/cpufreq/intel_pstate.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 5a0eeb84d3821..b2c60c4931dcd 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -2908,8 +2908,23 @@ static int intel_pstate_set_policy(struct cpufreq_policy *policy)
 		 */
 		intel_pstate_clear_update_util_hook(policy->cpu);
 		intel_pstate_set_pstate(cpu, pstate);
+
+		/*
+		 * Report the exact pinned frequency instead of the floor:
+		 * the CPU is pinned to pstate here and nothing else changes
+		 * it, unlike the general case below.
+		 */
+		policy->cur = pstate * cpu->pstate.scaling;
 	} else {
 		intel_pstate_set_update_util_hook(policy->cpu);
+
+		/*
+		 * Keep policy->cur within limits here: outside of the pinned
+		 * CPUFREQ_POLICY_PERFORMANCE case above, it is never updated
+		 * by the intel_pstate driver, but it is used as a stale
+		 * frequency value.
+		 */
+		policy->cur = policy->min;
 	}
 
 	if (hwp_active) {
@@ -2922,11 +2937,6 @@ static int intel_pstate_set_policy(struct cpufreq_policy *policy)
 			intel_pstate_clear_update_util_hook(policy->cpu);
 		intel_pstate_hwp_set(policy->cpu);
 	}
-	/*
-	 * policy->cur is never updated with the intel_pstate driver, but it
-	 * is used as a stale frequency value. So, keep it within limits.
-	 */
-	policy->cur = policy->min;
 
 	mutex_unlock(&intel_pstate_limits_lock);
 

---
base-commit: 502d801f0ab03e4f32f9a33d203154ce84887921
change-id: 20260729-bug-intel-pstate-policy-cur-1e1498a56641

Best regards,
-- 
Jing Wu <realwujing@xxxxxxxxx>