[PATCH v3 2/2] cpufreq: governor: Fix stale prev_cpu_nice spike when enabling ignore_nice_load

[Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>]

When ignore_nice_load is toggled from 0 to 1 via sysfs, dbs_update() may
run concurrently and observe the new tunable value while prev_cpu_nice
still holds a stale baseline, producing a spurious massive idle_time that
results in an incorrect CPU load value.

The race can be illustrated with two concurrent paths:

Path A (sysfs write, holds attr_set->update_lock):

governor_store()
  mutex_lock(&attr_set->update_lock)
  ignore_nice_load_store()
    dbs_data->ignore_nice_load = 1              /* (A1) */
    gov_update_cpu_data(dbs_data)
      mutex_lock(&policy_dbs->update_mutex)     /* (A2) */
        j_cdbs->prev_cpu_nice = kcpustat_field(...)
      mutex_unlock(&policy_dbs->update_mutex)
  mutex_unlock(&attr_set->update_lock)

Path B (work queue, wins the race between A1 and A2):

dbs_work_handler()
  mutex_lock(&policy_dbs->update_mutex)         /* acquired before A2 */
  dbs_update()
    ignore_nice = dbs_data->ignore_nice_load    /* sees new value: 1 */
    cur_nice = kcpustat_field(...)
    idle_time += div_u64(cur_nice - j_cdbs->prev_cpu_nice, ..) /* stale */
    j_cdbs->prev_cpu_nice = cur_nice
  mutex_unlock(&policy_dbs->update_mutex)

Fix this by unconditionally sampling cur_nice and advancing prev_cpu_nice
in dbs_update() on every call, regardless of ignore_nice. With
prev_cpu_nice always reflecting the most recent sample, enabling
ignore_nice_load can never produce a stale-baseline spike: the delta will
always be the nice time accumulated in the last sampling interval, not
since boot. The additional kcpustat_field() call per CPU per sample is
negligible given that the sampling path already reads idle and load
accounting.

To keep prev_cpu_nice handling consistent with the always-tracking
semantics introduced above:

  - gov_update_cpu_data() unconditionally resets prev_cpu_nice alongside
    prev_cpu_idle, so both baselines share the same timestamp when
    io_is_busy changes.  This prevents an interval mismatch between
    idle_time and nice_delta on the next dbs_update() when
    ignore_nice_load is enabled.
  - cpufreq_dbs_governor_start() unconditionally initializes prev_cpu_nice
    so the baseline is always valid from the first dbs_update() call;
    remove the ignore_nice guard and the now-unused ignore_nice variable.

Fixes: ee88415caf736b ("[CPUFREQ] Cleanup locking in conservative governor")
Fixes: 5a75c82828e7c0 ("[CPUFREQ] Cleanup locking in ondemand governor")
Fixes: 326c86deaed54a ("[CPUFREQ] Remove unneeded locks")
Signed-off-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
---
 drivers/cpufreq/cpufreq_governor.c | 33 +++++++++++++++++++-----------
 1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
index fc6f705c5a9c..8a85bd32defe 100644
--- a/drivers/cpufreq/cpufreq_governor.c
+++ b/drivers/cpufreq/cpufreq_governor.c
@@ -92,6 +92,12 @@ EXPORT_SYMBOL_GPL(sampling_rate_store);
  *
  * Call under the @dbs_data->attr_set.update_lock. The per-policy
  * update_mutex is acquired and released internally for each policy.
+ *
+ * Note: prev_cpu_nice is reset here unconditionally alongside prev_cpu_idle.
+ * When io_is_busy changes, both baselines must be advanced to the same
+ * timestamp so that the next dbs_update() computes idle_time and nice_delta
+ * over the same interval, preventing an artificially inflated idle_time when
+ * ignore_nice_load is enabled.
  */
 void gov_update_cpu_data(struct dbs_data *dbs_data)
 {
@@ -106,8 +112,7 @@ void gov_update_cpu_data(struct dbs_data *dbs_data)
 
 			j_cdbs->prev_cpu_idle = get_cpu_idle_time(j, &j_cdbs->prev_update_time,
 								  dbs_data->io_is_busy);
-			if (dbs_data->ignore_nice_load)
-				j_cdbs->prev_cpu_nice = kcpustat_field(&kcpustat_cpu(j), CPUTIME_NICE, j);
+			j_cdbs->prev_cpu_nice = kcpustat_field(&kcpustat_cpu(j), CPUTIME_NICE, j);
 		}
 		mutex_unlock(&policy_dbs->update_mutex);
 	}
@@ -121,6 +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;
+	u64 cur_nice;
 
 	/*
 	 * Sometimes governors may use an additional multiplier to increase
@@ -167,12 +173,18 @@ unsigned int dbs_update(struct cpufreq_policy *policy)
 
 		j_cdbs->prev_cpu_idle = cur_idle_time;
 
-		if (ignore_nice) {
-			u64 cur_nice = kcpustat_field(&kcpustat_cpu(j), CPUTIME_NICE, j);
-
+		/*
+		 * Always sample cur_nice and advance prev_cpu_nice, regardless
+		 * of ignore_nice.  This keeps prev_cpu_nice current so that
+		 * enabling ignore_nice_load via sysfs never produces a
+		 * stale-baseline spike (the delta will be at most one sampling
+		 * interval of accumulated nice time, not since boot).
+		 */
+		cur_nice = kcpustat_field(&kcpustat_cpu(j), CPUTIME_NICE, j);
+		if (ignore_nice)
 			idle_time += div_u64(cur_nice - j_cdbs->prev_cpu_nice, NSEC_PER_USEC);
-			j_cdbs->prev_cpu_nice = cur_nice;
-		}
+
+		j_cdbs->prev_cpu_nice = cur_nice;
 
 		if (unlikely(!time_elapsed)) {
 			/*
@@ -519,7 +531,7 @@ int cpufreq_dbs_governor_start(struct cpufreq_policy *policy)
 	struct dbs_governor *gov = dbs_governor_of(policy);
 	struct policy_dbs_info *policy_dbs = policy->governor_data;
 	struct dbs_data *dbs_data = policy_dbs->dbs_data;
-	unsigned int sampling_rate, ignore_nice, j;
+	unsigned int sampling_rate, j;
 	unsigned int io_busy;
 
 	if (!policy->cur)
@@ -529,7 +541,6 @@ int cpufreq_dbs_governor_start(struct cpufreq_policy *policy)
 	policy_dbs->rate_mult = 1;
 
 	sampling_rate = dbs_data->sampling_rate;
-	ignore_nice = dbs_data->ignore_nice_load;
 
 	mutex_lock(&policy_dbs->update_mutex);
 	io_busy = dbs_data->io_is_busy;
@@ -541,9 +552,7 @@ int cpufreq_dbs_governor_start(struct cpufreq_policy *policy)
 		 * Make the first invocation of dbs_update() compute the load.
 		 */
 		j_cdbs->prev_load = 0;
-
-		if (ignore_nice)
-			j_cdbs->prev_cpu_nice = kcpustat_field(&kcpustat_cpu(j), CPUTIME_NICE, j);
+		j_cdbs->prev_cpu_nice = kcpustat_field(&kcpustat_cpu(j), CPUTIME_NICE, j);
 	}
 	mutex_unlock(&policy_dbs->update_mutex);
 
-- 
2.43.0