Re: [PATCH v2 1/4] cpufreq: Fix re-boost issue after hotplugging a cpu

[zhenglifeng (A)]

On 2025/1/20 16:27, Viresh Kumar wrote:

> Hi,
> 
> I am bit confused by the sequence of events here and need some
> clarification. Lets assume that CPU can go from 1 GHz to 1.5 GHz
> without boost enabled and with boost it can go to 2 GHz.
> 
> On 17-01-25, 18:14, Lifeng Zheng wrote:
>> It turns out that cpuX will stay on the base frequency after performing
>> these operations:
>>
>> 1. boost all cpus: echo 1 > /sys/devices/system/cpu/cpufreq/boost
> 
> Boost enabled here, max_freq_req = 2 GHz.
> 
>> 2. offline the cpu: echo 0 > /sys/devices/system/cpu/cpuX/online
>>
>> 3. deboost all cpus: echo 0 > /sys/devices/system/cpu/cpufreq/boost
>>
>> 4. online the cpu: echo 1 > /sys/devices/system/cpu/cpuX/online
> 
> Boost is disabled currently here, but max_freq_req = 2 GHz, which is
> incorrect and the current change you are proposing fixes it I think.
> But it is not what you are claiming to fix.

Since boost is disabled, policy->max and policy->cpuinfo.max_freq will be
1.5GHz, this limits the actual frequency of the final.

> 
>> 5. boost all cpus again: echo 1 > /sys/devices/system/cpu/cpufreq/boost
> 
> Boost enabled again here, and max_freq_req = 2 GHz is the correct
> value.

In freq_qos_update_request(), if req->pnode.prio == new_value, it will
directly return 0 and not excecute freq_qos_apply(), in which will refresh
frequency. So the frequency will stay on base.

> 
> So the CPU doesn't stay at base frequency here, but 2 GHz only.
> 
>> This is because max_freq_req of the policy is not updated during the online
>> process, and the value of max_freq_req before the last offline is retained.
> 
> which was 2 GHz in your example.
> 
>> When the CPU is boosted again, freq_qos_update_request() will do nothing
>> because the old value is the same as the new one. This causes the CPU stay
>> on the base frequency. Update max_freq_req  in cpufreq_online() will solve
>> this problem.
>