linux-kernel - Re: [PATCH v1 1/5] cpuidle: governors: teo: Avoid selecting states with zero-size bins

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Message-ID: <02693dc9-e6f7-4683-8bba-4de5e8854689@arm.com>
Date: Mon, 26 Jan 2026 12:05:28 +0000
From: Christian Loehle <christian.loehle@....com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Linux PM <linux-pm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
 Doug Smythies <dsmythies@...us.net>
Subject: Re: [PATCH v1 1/5] cpuidle: governors: teo: Avoid selecting states
 with zero-size bins

On 1/26/26 11:40, Rafael J. Wysocki wrote:
> On Mon, Jan 26, 2026 at 10:18 AM Christian Loehle
> <christian.loehle@....com> wrote:
>>
>> On 1/23/26 20:46, Rafael J. Wysocki wrote:
>>> On Wed, Jan 21, 2026 at 2:10 PM Christian Loehle
>>> <christian.loehle@....com> wrote:
>>>>
>>>> On 1/14/26 19:44, Rafael J. Wysocki wrote:
>>>>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>>>>
>>>>> If the last two enabled idle states have the same target residency which
>>>>> is at least equal to TICK_NSET, teo may select the next-to-last one even
>>>>
>>>> s/TICK_NSET/TICK_NSEC
>>>
>>> Yup, thanks!
>>>
>>>>> though the size of that state's bin is 0, which is confusing.
>>>>>
>>>>> Prevent that from happening by adding a target residency check to the
>>>>> relevant code path.
>>>>>
>>>>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>>>> ---
>>>>>  drivers/cpuidle/governors/teo.c |   10 ++++++++++
>>>>>  1 file changed, 10 insertions(+)
>>>>>
>>>>> --- a/drivers/cpuidle/governors/teo.c
>>>>> +++ b/drivers/cpuidle/governors/teo.c
>>>>> @@ -388,6 +388,15 @@ static int teo_select(struct cpuidle_dri
>>>>>                       while (min_idx < idx &&
>>>>>                              drv->states[min_idx].target_residency_ns < TICK_NSEC)
>>>>>                               min_idx++;
>>>>> +
>>>>> +                     /*
>>>>> +                      * Avoid selecting a state with a lower index, but with
>>>>> +                      * the same target residency as the current candidate
>>>>> +                      * one.
>>>>> +                      */
>>>>> +                     if (drv->states[min_idx].target_residency_ns ==
>>>>> +                                     drv->states[idx].target_residency_ns)
>>>>
>>>> We need to check that min_idx isn't disabled though, otherwise we now skip a
>>>> potential (enabled) idx==1 if min_idx==2 and min_idx is disabled.
>>>
>>> Not really because idx is the current candidate state and it is
>>> enabled.  We'll use idx if this check is true, not min_idx.
>>>
>>
>> Are you sure?
> 
> Yeah, pretty much.
> 
>> I meant initially:
>>
>>                 for (i = intercept_max_idx; i >= min_idx; i--) {
>>                         intercept_sum += cpu_data->state_bins[i].intercepts;
>>
>>                         if (dev->states_usage[i].disable)
>>                                 continue;
>>
>>                         idx = i;
>>                         if (2 * intercept_sum > idx_intercept_sum)
>>                                 break;
>>                 }
>> might skip an idx==3 if it enters with min_idx==2 (sorry, messed up the +-1 in the initial mail)
>> even though idx==3 might have the same residency as idx==2.
>> So if idx==2 is disabled we could've selected idx==3, but now won't and will go for idx==1 or
>> whatever is the next shallower enabled state.
> 
> But that's after patch [5/5] that has problems (I have a new version
> of it ready to send, will do that later today).
> 
> Look at the original 6.19-rc code and patch [1/1] by itself and you'll
> see what I mean.
> 
Duh you're right, sorry about the noise :/