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SubjectRe: [RFC PATCH v2 1/2] sched/fair: Introduce short duration task check
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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@intel.com>
> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Chen Yu <yu.c.chen@intel.com>
> ---
> 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.

> +}
> +
> /*
> * 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.

> + 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

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