cpufreq: governor: Fix stale prev_cpu_nice spike when enabling ignore_nice_load

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>
Link: https://patch.msgid.link/20260419132655.3800673-3-zhongqiu.han@oss.qualcomm.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

0 files changed, 0 insertions, 0 deletions