cpufreq: schedutil: set sg_policy->next_freq to the final cpufreq

Quoted 8 lines

From: zhuguangqing <[email protected]>

In the following code path, next_freq is clamped between policy->min and policy->max twice in functions cpufreq_driver_resolve_freq() and cpufreq_driver_fast_switch(). For there is no update_lock in the code path, policy->min and policy->max may be modified (one or more times), so sg_policy->next_freq updated in function sugov_update_next_freq() may be not the final cpufreq.

Understood until here, but not sure I followed everything after that.

Quoted 11 lines

Next time when we use
"if (sg_policy->next_freq == next_freq)" to judge whether to update next_freq, we may get a wrong result.

-> sugov_update_single()
-> get_next_freq()

    -> cpufreq_driver_resolve_freq()
  -> sugov_fast_switch()
    -> sugov_update_next_freq()
    -> cpufreq_driver_fast_switch()

Quoted 7 lines

For example, at first sg_policy->next_freq is 1 GHz, but the final cpufreq is 1.2 GHz because policy->min is modified to 1.2 GHz when we reached cpufreq_driver_fast_switch(). Then next time, policy->min is changed before we reached cpufreq_driver_resolve_freq() and (assume) next_freq is 1 GHz, we find "if (sg_policy->next_freq == next_freq)" is satisfied so we don't change the cpufreq. Actually we should change the cpufreq to 1.0 GHz this time.

FWIW, whenever policy->min/max gets changed, sg_policy->limits_changed gets set to true by sugov_limits() and the next time schedutil callback gets called from the scheduler, we will fix the frequency.

And so there shouldn't be any issue here, unless I am missing something.

--
viresh

Thanks for your comments. Maybe my description was not clear before.

If I understand correctly, when policy->min/max get changed in the time Window between get_next_freq() and sugov_fast_switch(), to be more precise between cpufreq_driver_resolve_freq() and cpufreq_driver_fast_switch(), the issue may happen.

For example, the first time schedutil callback gets called from the scheduler, we reached get_next_freq() and calculate the next_freq, suppose next_freq is 1.0 GHz, then sg_policy->next_freq is updated to 1.0 GHz in sugov_update_next_freq(). If policy->min/max get change right now, suppose policy->min is changed to 1.2 GHz, then the final next_freq is 1.2 GHz for there is another clamp between policy->min and policy->max in cpufreq_driver_fast_switch(). Then sg_policy->next_freq(1.0 GHz) is not the final next_freq(1.2 GHz).

The second time schedutil callback gets called from the scheduler, there are two issues:
(1) Suppose policy->min is still 1.2 GHz, we reached get_next_freq() and calculate the next_freq, because sg_policy->limits_changed gets set to true by sugov_limits() and there is a clamp between policy->min and policy->max, so this time next_freq will be greater than or equal to 1.2 GHz, suppose next_freq is also 1.2 GHz. Now next_freq is 1.2 GHz and sg_policy->next_freq is 1.0 GHz, then we find
"if (sg_policy->next_freq == next_freq)" is not satisfied and we call cpufreq driver to change the cpufreq to 1.2 GHz. Actually it's already 1.2 GHz, it's not necessary to change this time.

(2) Suppose policy->min was changed again to 1.0 GHz before, we reached get_next_freq() and calculate the next_freq, suppose next_freq is also 1.0 GHz. Now next_freq is 1.0 GHz and sg_policy->next_freq is also 1.0 GHz, then we find "if (sg_policy->next_freq == next_freq)" is satisfied and we don't change the cpufreq. Actually we should change the cpufreq to 1.0 GHz this time.