Re: [RFC PATCH v2 1/2] sched/fair: Introduce short duration task check

[Chen Yu]

Hi Abel,
On 2022-10-24 at 18:10:22 +0800, Abel Wu wrote:
> LGTM, and please check inline comments.
> 
> On 10/23/22 11:32 PM, Chen Yu wrote:
> > Introduce short-duration task checks, as there is a
> > 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.
> > 
> > PELT could decay the runtime time, so maybe something like
> > se.util_avg could be used. But after a second thought, PELT
> > might not be appropriate to measure the 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, it 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. The calculation of average duration is borrowed
> > from the idea of Exponential Weighted Moving Average (EWMA) in
> > util_est_update(). The effect is that the average task duration
> > is calculated by:
> > new_avg_duration = old_avg_duration * 0.875 + lastest_duration * 0.125;
> > which is what update_avg() does now.
> > 
> > Introduce SIS_SHORT to control this strategy.
> > 
> > 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   |  8 +++++
> >   kernel/sched/core.c     |  2 ++
> >   kernel/sched/fair.c     | 77 +++++++++++++++++++++++++++++++++++++++++
> >   kernel/sched/features.h |  1 +
> >   4 files changed, 88 insertions(+)
> > 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index ffb6eb55cd13..ff74301b1548 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -557,6 +557,14 @@ struct sched_entity {
> >   	u64				prev_sum_exec_runtime;
> >   	u64				nr_migrations;
> > +	/*
> > +	 * The 'snapshot' of sum_exec_runtime when task
> > +	 * voluntarily switches out. This is used to
> > +	 * calculate the average duration below.
> > +	 */
> > +	u64				prev_sum_runtime_vol;
> > +	/* average duration of a task */
> > +	u64				dur_avg;
> >   #ifdef CONFIG_FAIR_GROUP_SCHED
> >   	int				depth;
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 5800b0623ff3..f5281fe7f8f0 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4330,6 +4330,8 @@ 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_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..8820d0d14519 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6200,6 +6200,19 @@ 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.
> > + * sysctl_sched_min_granularity is chosen as the threshold,
> > + * as this value is the minimal slice if there are too many
> > + * runnable tasks, see __sched_period().
> > + */
> > +static inline int is_short_task(struct task_struct *p)
> > +{
> > +	return sched_feat(SIS_SHORT) &&
> > +		(p->se.dur_avg <= sysctl_sched_min_granularity);
> 
> The min_granularity is default to 750us, and can be tuned higher on
> machines running batch workloads. So I'm not sure it is appropriate
> to define 'short' by min_granularity.
>
Yes, it seems that 'short' is a subjective term, different user/system
might have different defination of 'short' task. For a system that set
a high sysctl_sched_min_granularity, the admin might expect that most CFS
tasks should last longer. That is to say, if a task's duration is 1ms when
sysctl_sched_min_granularity is 750us, it is not a short task. But if we
enlarge the sysctl_sched_min_granularity to 2ms, this task becomes a short
one, when compared to other CFS tasks in the system. You are right binding
the 'short duration' with sysctl_sched_min_granularity might look subjective,
but I can not find other existing metrics(also configurable by userspace)
in the kernel to describe this.
> > +}
> > +
> >   /*
> >    * The purpose of wake_affine() is to quickly determine on which CPU we can run
> >    * soonest. For the purpose of speed we only consider the waking and previous
> > @@ -7679,6 +7692,70 @@ static void put_prev_task_fair(struct rq *rq, struct task_struct *prev)
> >   {
> >   	struct sched_entity *se = &prev->se;
> >   	struct cfs_rq *cfs_rq;
> > +	u64 this_dur_avg, last_dur_avg;
> > +	long delta;
> > +
> > +	/*
> > +	 * Calculate the task's average duration.
> > +	 *
> > +	 * Only consider that task voluntarily relinquishes the CPU.
> > +	 * For example, suppose on CPU1, task p1 and p2 runs
> > +	 * alternatively:
> > +	 *
> > +	 * --------------------> time
> > +	 *
> > +	 * | p1 runs 1ms | p2 preempt p1 | p1 runs 0.5ms and sleeps |
> > +	 * ^             ^               ^                          ^
> > +	 * |_____________|               |__________________________|
> > +	 *        |____________________________________|
> > +	 *                   p1's duration
> > +	 *
> > +	 *
> > +	 * The duration of p1 is 1.5ms rather than 0.5ms or 1ms
> > +	 * in above case. That is to say, the duration period starts
> > +	 * when task p1 switches in, ends when task p1 voluntarily
> > +	 * relinquishes the CPU. This duration descibes the "nature"
> > +	 * of a task: If a task is not preempted, how long it will
> > +	 * run.
> > +	 *
> > +	 * The Exponential Weighted Moving Average (EWMA)
> > +	 * is used to calculate the average duration.
> > +	 * Borrowed from util_est_update():
> > +	 *
> > +	 *  ewma(t) = w * this_dur_avg + (1-w) * ewma(t-1)
> > +	 *
> > +	 * When 'w' is 0.125, it becomes update_avg().
> > +	 * This indicates that we care about approximately
> > +	 * the recent 1 / 0.125 = 8 history duration.
> > +	 */
> > +	if (sched_feat(SIS_SHORT) && !prev->on_rq) {
> > +		/*
> > +		 * sum_exec_runtime has been updated in update_curr()
> > +		 * because we reach here via dequeue.
> > +		 */
> > +		this_dur_avg = se->sum_exec_runtime - se->prev_sum_runtime_vol;
> > +		/*
> > +		 * Record the accumulated runtime when task voluntarily
> > +		 * switches out. End of old duration period, a new period
> > +		 * starts.
> > +		 */
> > +		se->prev_sum_runtime_vol = se->sum_exec_runtime;
> > +
> > +		last_dur_avg = se->dur_avg;
> > +		delta = this_dur_avg - last_dur_avg;
> > +		/* consider large change to avoid frequent update */
> > +		if (abs(delta) >= sysctl_sched_min_granularity) {
> > +			/*
> > +			 * If it is the first time the task starts to
> > +			 * record dur_avg, discard the initial value 0.
> > +			 * Otherwise, calculate the EWMA.
> > +			 */
> > +			if (unlikely(!this_dur_avg))
> > +				se->dur_avg = this_dur_avg;
> 
> Would it better initialize to 'short' (min_granularity in this case)?
> So we can get rid of this check.
> 
Good suggestion, the task is initilized as 'non-short', and
is_short_task() needs to be modified from "<=" to "<".

thanks,
Chenyu
> > +			else
> > +				update_avg(&se->dur_avg, this_dur_avg);
> > +		}
> > +	}
> >   	for_each_sched_entity(se) {
> >   		cfs_rq = cfs_rq_of(se);
> > diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> > index ee7f23c76bd3..efdc29c42161 100644
> > --- a/kernel/sched/features.h
> > +++ b/kernel/sched/features.h
> > @@ -62,6 +62,7 @@ SCHED_FEAT(TTWU_QUEUE, true)
> >    */
> >   SCHED_FEAT(SIS_PROP, false)
> >   SCHED_FEAT(SIS_UTIL, true)
> > +SCHED_FEAT(SIS_SHORT, true)
> >   /*
> >    * Issue a WARN when we do multiple update_rq_clock() calls