Re: [PATCH v3 1/2] sched/fair: Introduce short duration task check

[Joel Fernandes]

> On Dec 3, 2022, at 2:50 AM, Chen Yu <yu.c.chen@xxxxxxxxx> wrote:
> 
> Hi Honglei,
>> On 2022-12-02 at 15:44:18 +0800, Honglei Wang wrote:
>> 
>> 
>>> On 2022/12/1 16:44, Chen Yu wrote:
>>> Introduce short-duration task checks, as there is requirement
>>> to leverage this attribute for better task placement.
>>> 
>>> There are several choices of metrics that could be used to
>>> indicate if a task is a short-duration task.
>>> 
>>> At first thought the (p->se.sum_exec_runtime / p->nvcsw)
>>> could be used to measure the task duration. However, the
>>> history long past was factored too heavily in such a formula.
>>> Ideally, the old activity should decay and not affect
>>> the current status too much.
>>> 
>>> Although something based on PELT could be used, se.util_avg might
>>> not be appropriate to describe the task duration:
>>> 1. Task p1 and task p2 are doing frequent ping-pong scheduling on
>>>    one CPU, both p1 and p2 have a short duration, but the util_avg
>>>    can be up to 50%.
>>> 2. Suppose a task lasting less than 4ms is regarded as a short task.
>>>    If task p3 runs for 6ms and sleeps for 32ms, p3 should not be a
>>>    short-duration task. However, PELT would decay p3's accumulated
>>>    running time from 6ms to 3ms, because 32ms is the half-life in PELT.
>>>    As a result, p3 would be incorrectly treated as a short task.
>>> 
>>> It was found that there was once a similar feature to track the
>>> duration of a task, which is in Commit ad4b78bbcbab ("sched: Add
>>> new wakeup preemption mode: WAKEUP_RUNNING"). Unfortunately, it
>>> was reverted because it was an experiment. So pick the patch up
>>> again, by recording the average duration when a task voluntarily
>>> switches out. Introduce SIS_SHORT to control this strategy.
>>> 
>>> The threshold of short duration reuses sysctl_sched_min_granularity,
>>> so it can be tuned by the user. Ideally there should be a dedicated
>>> parameter for the threshold, but that might introduce complexity.
>>> 
>>> Suggested-by: Tim Chen <tim.c.chen@xxxxxxxxx>
>>> Suggested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>>> Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
>>> ---
>>>  include/linux/sched.h   |  4 ++++
>>>  kernel/sched/core.c     |  2 ++
>>>  kernel/sched/fair.c     | 17 +++++++++++++++++
>>>  kernel/sched/features.h |  1 +
>>>  4 files changed, 24 insertions(+)
>>> 
>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>>> index ffb6eb55cd13..64b7acb77a11 100644
>>> --- a/include/linux/sched.h
>>> +++ b/include/linux/sched.h
>>> @@ -558,6 +558,10 @@ struct sched_entity {
>>>      u64                nr_migrations;
>>> +    u64                prev_sum_exec_runtime_vol;
>>> +    /* average duration of a task */
>>> +    u64                dur_avg;
>>> +
>>>  #ifdef CONFIG_FAIR_GROUP_SCHED
>>>      int                depth;
>>>      struct sched_entity        *parent;
>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>> index daff72f00385..c5202f1be3f7 100644
>>> --- a/kernel/sched/core.c
>>> +++ b/kernel/sched/core.c
>>> @@ -4348,6 +4348,8 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
>>>      p->se.prev_sum_exec_runtime    = 0;
>>>      p->se.nr_migrations        = 0;
>>>      p->se.vruntime            = 0;
>>> +    p->se.dur_avg            = 0;
>>> +    p->se.prev_sum_exec_runtime_vol    = 0;
>>>      INIT_LIST_HEAD(&p->se.group_node);
>>>  #ifdef CONFIG_FAIR_GROUP_SCHED
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index e4a0b8bd941c..a4b314b664f8 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -6200,6 +6200,16 @@ static int wake_wide(struct task_struct *p)
>>>      return 1;
>>>  }
>>> +/*
>>> + * If a task switches in and then voluntarily relinquishes the
>>> + * CPU quickly, it is regarded as a short duration task.
>>> + */
>>> +static inline int is_short_task(struct task_struct *p)
>>> +{
>>> +    return sched_feat(SIS_SHORT) &&
>>> +        (p->se.dur_avg <= sysctl_sched_min_granularity);
>>> +}
>>> +
>> 
>> Hi Yu,
>> 
>> I still have a bit concern about the sysctl_sched_min_granularity stuff..
>> This grab can be set to different value which will impact the action of this
>> patch and make things not totally under control.

There are already ways to misconfigure sched sysctl to make bad/weird things happen.

>> Not sure if we can add a new grab for this.. The test result shows good
>> improvement for short task, and with this grab, admins will be able to
>> custom the system base on their own 'short task' view.
>> 
> It would be ideal to have a dedicated parameter to tweak this. For example,
> something under /sys/kernel/debug/sched/, and initilized to sysctl_sched_min_granularity
> by default. 

It would be nice to not have to introduce a new knob for this. IMO, min_granularity is reasonable.

Thanks.



> 
> thanks,
> Chenyu