OLK-6.6: cpufreq: governor: Fix negative 'idle_time' handling in dbs_update()

We observed an issue that the CPU frequency can't raise up with a 100% CPU
load when NOHZ is off and the 'conservative' governor is selected.

'idle_time' can be negative if it's obtained from get_cpu_idle_time_jiffy()
when NOHZ is off.  This was found and explained in commit 9485e4ca0b48
("cpufreq: governor: Fix handling of special cases in dbs_update()").

However, commit 7592019634f8 ("cpufreq: governors: Fix long idle detection
logic in load calculation") introduced a comparison between 'idle_time' and
'samling_rate' to detect a long idle interval.  While 'idle_time' is
converted to int before comparison, it's actually promoted to unsigned
again when compared with an unsigned 'sampling_rate'.  Hence, this leads to
wrong idle interval detection when it's in fact 100% busy and sets
policy_dbs->idle_periods to a very large value.  'conservative' adjusts the
frequency to minimum because of the large 'idle_periods', such that the
frequency can't raise up.  'Ondemand' doesn't use policy_dbs->idle_periods
so it fortunately avoids the issue.

Correct negative 'idle_time' to 0 before any use of it in dbs_update().

GVP openEuler/kernel
Closed

Content Risk Flag

Comments (3)

GVPopenEuler/kernelClosed

Content Risk Flag