linux-kernel - Re: [PATCH] sched/fair: Prefer small idle cores for forkees

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Date:   Tue, 25 Jan 2022 12:43:57 +0530
From:   Chitti Babu Theegala <quic_ctheegal@...cinc.com>
To:     Vincent Donnefort <vincent.donnefort@....com>
CC:     <mingo@...hat.com>, <peterz@...radead.org>,
        <juri.lelli@...hat.com>, <vincent.guittot@...aro.org>,
        <dietmar.eggemann@....com>, <rostedt@...dmis.org>,
        <joel@...lfernandes.org>, <linux-arm-msm@...r.kernel.org>,
        <quic_lingutla@...cinc.com>, <linux-kernel@...r.kernel.org>,
        <quic_rjendra@...cinc.com>
Subject: Re: [PATCH] sched/fair: Prefer small idle cores for forkees


On 1/21/2022 3:47 PM, Vincent Donnefort wrote:
> On Thu, Jan 20, 2022 at 10:15:07PM +0530, Chitti Babu Theegala wrote:
>>
>>
>> On 1/13/2022 10:05 PM, Vincent Donnefort wrote:
>>> On Wed, Jan 12, 2022 at 08:09:02PM +0530, Chitti Babu Theegala wrote:
>>>> Newly forked threads don't have any useful utilization data yet and
>>>> it's not possible to forecast their impact on energy consumption.
>>>> update_pick_idlest These forkees (though very small, most times) end up waking big
>>>> cores from deep sleep for that very small durations.
>>>>
>>>> Bias all forkees to small cores to prevent waking big cores from deep
>>>> sleep to save power.
>>>
>>> This bias might be interesting for some workloads, but what about the
>>> others? (see find_energy_efficient_cpu() comment, which discusses forkees).
>>>
>>
>> Yes, I agree with the find_energy_efficient_cpu() comment that we don't have
>> any useful utilization data yet and hence not possible to forecast. However,
>> I don't see any point in penalizing the power by waking up bigger cores
>> which are in deep sleep state for very small workloads.
>>
>> This patch helps lighter workloads during idle conditions w.r.t power POV.
>> For active (interactive or heavier) workloads, on most big.Little systems'
>> these foreground tasks get pulled into gold affined cpu-sets where this
>> patch would not play any spoilsport. Even for systems with such cpu-sets not
>> defined, heavy workloads might need just another 1 or 2 scheduling windows
>> for ramping to better freq or core.
> 
> Scheduling windows? I suppose you do not refer to PELT here, so I'm not sure
> this argument applies here.

Sorry. I didn’t mean it to be WALT. I meant that ramp up would happen in 
next couple of ms which can give very small penalty for such heavy 
workloads for the initial ms.

> 
> Beside, CFS always bias toward performance (except feec(), which does it in a
> lesser extent).
> 

Yes, aware that CFS is perf bias. Can we have a knob atleast which can 
turn-on such power friendly features ?

>>
>>>>
>>>> Signed-off-by: Chitti Babu Theegala <quic_ctheegal@...cinc.com>
>>>> ---
>>>>    kernel/sched/fair.c | 16 +++++++++++-----
>>>>    1 file changed, 11 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index 6e476f6..d407bbc 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -5976,7 +5976,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
>>>>    }
>>>>    static struct sched_group *
>>>> -find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu);
>>>> +find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu, int sd_flag);
>>>>    /*
>>>>     * find_idlest_group_cpu - find the idlest CPU among the CPUs in the group.
>>>> @@ -6063,7 +6063,7 @@ static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p
>>>>    			continue;
>>>>    		}
>>>> -		group = find_idlest_group(sd, p, cpu);
>>>> +		group = find_idlest_group(sd, p, cpu, sd_flag);
>>>>    		if (!group) {
>>>>    			sd = sd->child;
>>>>    			continue;
>>>> @@ -8997,7 +8997,8 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd,
>>>>    static bool update_pick_idlest(struct sched_group *idlest,
>>>>    			       struct sg_lb_stats *idlest_sgs,
>>>>    			       struct sched_group *group,
>>>> -			       struct sg_lb_stats *sgs)
>>>> +			       struct sg_lb_stats *sgs,
>>>> +			       int sd_flag)
>>>>    {
>>>>    	if (sgs->group_type < idlest_sgs->group_type)
>>>>    		return true;
>>>> @@ -9034,6 +9035,11 @@ static bool update_pick_idlest(struct sched_group *idlest,
>>>>    		if (idlest_sgs->idle_cpus > sgs->idle_cpus)
>>>>    			return false;
>>>> +		/* Select smaller cpu group for newly woken up forkees */
>>>> +		if ((sd_flag & SD_BALANCE_FORK) && (idlest_sgs->idle_cpus &&
>>>> +			!capacity_greater(idlest->sgc->max_capacity, group->sgc->max_capacity)))
>>>> +			return false;
>>>> +
>>>
>>> Energy biased placement should probably be applied only when EAS is enabled.
>>>
>>> It's especially true here, if all CPUs have the same capacity, capacity_greater
>>> would be always false. So unless I missed something, we wouldn't let the group_util
>>> evaluation happen, would we?
>>
>> True. I am uploading new version patch with a EAS enablement check in place.
>>
>>>
>>> [...]