RE: Ask for help on governor

["Doug Smythies" <dsmythies@telus.net>]

On 2017.12.15 07:54 Rafael J. Wysocki wrote:
> On Friday, December 15, 2017 8:37:38 AM Doug Smythies wrote:
>
>> Could you please try the below reversion patch.
>> It is on top of kernel 4.15-rc3.
>> 
>> On my system, and for the intel_cpufrequ scaling driver,
>> the default sampling_rate goes back to 20000 (which works)
>> from 500 uSec (which doesn't), for both the conservative
>> and ondemand governors.
>
> OK
> 
> So AFAICS the problem is that dbs_update() makes assumptions that are not
> me, quite obviously, if the sampling interval is less than the tick period.
>
> For example, the "time_elapsed > 2 * sampling_rate" condition in it may
> very well trigger every time in that case and that obviously affects the
> conservative governor.

It does trigger every time and that is why the conservative gov. sticks at
low frequency.

> What about the patch below, then?

I am testing now, from previous work, and already reported on this thread,
I already know that the conservative governor will be O.K..
It will take me awhile to thoroughly test the ondemand governor. At only 1.0
ticks at best it will be quite noisy and more sensitive to work/sleep frequencies
for periodic workflows.

By the way, I modified your proposed patch, which needed to divide by
NSEC_PER_USEC instead of multiply by.
i.e. I am testing this:

diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
index 58d4f4e..1c2fd1d 100644
--- a/drivers/cpufreq/cpufreq_governor.c
+++ b/drivers/cpufreq/cpufreq_governor.c
@@ -430,7 +430,14 @@ int cpufreq_dbs_governor_init(struct cpufreq_policy *policy)
        if (ret)
                goto free_policy_dbs_info;

+       /*
+        * The sampling interval should not be greater than the transition
+        * latency of the CPU, but it also cannot be less than the tick period
+        * for dbs_update() to work correctly.
+        */
        dbs_data->sampling_rate = cpufreq_policy_transition_delay_us(policy);
+       if (dbs_data->sampling_rate < TICK_NSEC / NSEC_PER_USEC)
+               dbs_data->sampling_rate = TICK_NSEC / NSEC_PER_USEC;

        if (!have_governor_per_policy())
                gov->gdbs_data = dbs_data;

... Doug