linux-kernel - Re: [PATCH] cpuidle: menu: Optimize bucket assignment when next_timer_ns equals KTIME

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Message-ID: <864cf1e3-25c2-42d0-845e-1bb1cfce3802@arm.com>
Date: Thu, 3 Apr 2025 11:06:31 +0100
From: Christian Loehle <christian.loehle@....com>
To: Zhongqiu Han <quic_zhonhan@...cinc.com>, rafael@...nel.org,
 daniel.lezcano@...aro.org
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpuidle: menu: Optimize bucket assignment when
 next_timer_ns equals KTIME_MAX

On 4/3/25 11:01, Zhongqiu Han wrote:
> On 4/3/2025 5:34 PM, Christian Loehle wrote:
>> On 4/3/25 10:28, Zhongqiu Han wrote:
>>> Directly assign the last bucket value instead of calling which_bucket()
>>> when next_timer_ns equals KTIME_MAX, the largest possible value that
>>> always falls into the last bucket. This avoids unnecessary calculations
>>> and enhances performance.
>>>
>>> Signed-off-by: Zhongqiu Han <quic_zhonhan@...cinc.com>
>>
>> Reviewed-by: Christian Loehle <christian.loehle@....com>
>>
>>> ---
>>>   drivers/cpuidle/governors/menu.c | 7 ++++++-
>>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
>>> index 39aa0aea61c6..8fc7fbed0052 100644
>>> --- a/drivers/cpuidle/governors/menu.c
>>> +++ b/drivers/cpuidle/governors/menu.c
>>> @@ -255,7 +255,12 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
>>>            */
>>>           data->next_timer_ns = KTIME_MAX;
>>>           delta_tick = TICK_NSEC / 2;
>>> -        data->bucket = which_bucket(KTIME_MAX);
>>> +        /*
>>> +         * Assign the last bucket value directly instead of calling
>>> +         * which_bucket(), since KTIME_MAX is the largest possible
>>> +         * value that always falls into the last bucket.
>>> +         */
>>
>> comment almost seems overkill.
>>
>>> +        data->bucket = BUCKETS - 1;
>>>       }
>>>         if (unlikely(drv->state_count <= 1 || latency_req == 0) ||
>>
> Thanks Christian for the review~
> 
> Actually I just want to add a comment to indicate that which_bucket()
> was once called here, in case which_bucket() changes in the future,
> and however, we stayed with the original approach, leading to the
> inconsistency.
> 
> Could you please review the comment below and let me know if it's okay
> or if I should not add any log? Thanks a lot~
> 
> /* KTIME_MAX falls into the last bucket, skip which_bucket(). */
> 
> 
> 
> I will collect review comments before arise patch V2.

Honestly I'd be fine without a comment, it's pretty obvious that
everything containing "bucket =" needs to be changed if the bucket
logic ever changes.