linux-kernel - [PATCH] cpufreq: conservative: Fix requested

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Date:   Mon,  8 Oct 2018 17:09:00 +0200
From:   Waldemar Rymarkiewicz <waldemar.rymarkiewicz@...il.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Waldemar Rymarkiewicz <waldemarx.rymarkiewicz@...el.com>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] cpufreq: conservative: Fix requested_freq handling

From: Waldemar Rymarkiewicz <waldemarx.rymarkiewicz@...el.com>

The governor updates dbs_info->requested_freq only after increasing or
decreasing frequency. There is, however, an use case when this is not
sufficient.

Imagine, external module constraining cpufreq policy in a way that policy->max
= policy->min = max_available_freq (eg. 1Ghz). CPUfreq will set freq to
max_freq and conservative gov will not try downscale/upscale due to the
limits. It will just exit instead

    if (requested_freq > policy->max || requested_freq < policy->min)
            //max=min=1Ghz -> requested_freq=cur=1Ghz
            requested_freq = policy->cur;
    [...]
    if (requested_freq == policy->max)
             goto out;

In a result, dbs_info->requested_freq is not updated with newly calculated
requested_freq=1Ghz. Next, execution of update routine will use again
previously stored requested_freq (in my case it was min_available_freq)

    [...]
    unsigned int requested_freq = dbs_info->requested_freq;
    [....]

Now, when external module returns to previous policy limits that is
policy->min = min_available_freq and policy->max = max_available_freq,
conservative governor is not able to decrease frequency because stored
requested_freq is still or rather already set to min_available_freq so
the check (for decreasing)

    [...]
    if (load < cs_tuners->down_threshold) {
    [....]
           if (requested_freq == policy->min)
                   goto out;
    [...]

returns from routine before it does any freq change. To fix that just update
dbs_info->requested_freq every time we go out from the update routine.

Signed-off-by: Waldemar Rymarkiewicz <waldemarx.rymarkiewicz@...el.com>
---
 drivers/cpufreq/cpufreq_conservative.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/cpufreq/cpufreq_conservative.c b/drivers/cpufreq/cpufreq_conservative.c
index f20f20a..7f90f6e 100644
--- a/drivers/cpufreq/cpufreq_conservative.c
+++ b/drivers/cpufreq/cpufreq_conservative.c
@@ -113,7 +113,6 @@ static unsigned int cs_dbs_update(struct cpufreq_policy *policy)
 			requested_freq = policy->max;
 
 		__cpufreq_driver_target(policy, requested_freq, CPUFREQ_RELATION_H);
-		dbs_info->requested_freq = requested_freq;
 		goto out;
 	}
 
@@ -136,10 +135,10 @@ static unsigned int cs_dbs_update(struct cpufreq_policy *policy)
 			requested_freq = policy->min;
 
 		__cpufreq_driver_target(policy, requested_freq, CPUFREQ_RELATION_L);
-		dbs_info->requested_freq = requested_freq;
 	}
 
  out:
+	dbs_info->requested_freq = requested_freq;
 	return dbs_data->sampling_rate;
 }
 
-- 
2.10.1