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Date: Sun, 23 Oct 2022 23:32:31 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Peter Zijlstra <peterz@...radead.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Tim Chen <tim.c.chen@...el.com>,
Mel Gorman <mgorman@...hsingularity.net>
Cc: Juri Lelli <juri.lelli@...hat.com>,
Rik van Riel <riel@...riel.com>,
Aaron Lu <aaron.lu@...el.com>,
Abel Wu <wuyun.abel@...edance.com>,
K Prateek Nayak <kprateek.nayak@....com>,
Yicong Yang <yangyicong@...ilicon.com>,
"Gautham R . Shenoy" <gautham.shenoy@....com>,
Ingo Molnar <mingo@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
Hillf Danton <hdanton@...a.com>,
Honglei Wang <wanghonglei@...ichuxing.com>,
Len Brown <len.brown@...el.com>,
Chen Yu <yu.chen.surf@...il.com>, linux-kernel@...r.kernel.org,
Chen Yu <yu.c.chen@...el.com>
Subject: [RFC PATCH v2 1/2] sched/fair: Introduce short duration task check
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@...el.com>
Suggested-by: Vincent Guittot <vincent.guittot@...aro.org>
Signed-off-by: Chen Yu <yu.c.chen@...el.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 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;
+ 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
--
2.25.1
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