Re: [PATCH V2 1/4] cpufreq: stats: Defer stats update to cpufreq_stats_record_transition()

[Lukasz Luba]

On 9/16/20 7:45 AM, Viresh Kumar wrote:

In order to prepare for lock-less stats update, add support to defer any
updates to it until cpufreq_stats_record_transition() is called.

Signed-off-by: Viresh Kumar <viresh.ku...@linaro.org>
---
  drivers/cpufreq/cpufreq_stats.c | 75 ++++++++++++++++++++++++---------
  1 file changed, 56 insertions(+), 19 deletions(-)

[snip]

@@ -228,10 +264,11 @@ void cpufreq_stats_record_transition(struct 
cpufreq_policy *policy,
        struct cpufreq_stats *stats = policy->stats;
        int old_index, new_index;

- if (!stats) {

-               pr_debug("%s: No stats found\n", __func__);
+       if (!stats)
                return;
-       }
+
+       if (READ_ONCE(stats->reset_pending))
+               cpufreq_stats_reset_table(stats);

This is in the hot path code, called from the scheduler. I wonder if we
avoid it or make that branch 'unlikely'?

if (unlikely(READ_ONCE(stats->reset_pending)))

Probably the CPU (when it has good prefetcher) would realize about it,
but maybe we can help a bit here.