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

[Chen Yu]

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.
> 
> 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. 
Hi Peter, Vincent,
may I have your opinion if this is applicable?

thanks,
Chenyu