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Date: Tue, 21 Mar 2023 10:36:49 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: wuchi <wuchi.zero@...il.com>
Cc: mingo@...hat.com, juri.lelli@...hat.com,
vincent.guittot@...aro.org, dietmar.eggemann@....com,
rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
bristot@...hat.com, vschneid@...hat.com,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/core: Reduce cost of sched_move_task when
config autogroup
On Tue, Mar 21, 2023 at 02:44:59PM +0800, wuchi wrote:
> kernel/sched/core.c | 30 ++++++++++++++++++++++++++++--
> 1 file changed, 28 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a380f34789a2..1e7d6a8c3455 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -10330,7 +10330,7 @@ void sched_release_group(struct task_group *tg)
> spin_unlock_irqrestore(&task_group_lock, flags);
> }
>
> -static void sched_change_group(struct task_struct *tsk)
> +static struct task_group *sched_get_task_group(struct task_struct *tsk)
> {
> struct task_group *tg;
>
> @@ -10342,7 +10342,28 @@ static void sched_change_group(struct task_struct *tsk)
> tg = container_of(task_css_check(tsk, cpu_cgrp_id, true),
> struct task_group, css);
> tg = autogroup_task_group(tsk, tg);
> - tsk->sched_task_group = tg;
> +
> + return tg;
> +}
> +
> +static bool sched_task_group_changed(struct task_struct *tsk)
> +{
> + /*
> + * Some sched_move_task calls are useless because that
> + * task_struct->sched_task_group maybe not changed (equals
> + * task_group of cpu_cgroup) when system enable autogroup.
> + * So do some checks in sched_move_task.
> + */
> +#ifdef CONFIG_SCHED_AUTOGROUP
> + return sched_get_task_group(tsk) != tsk->sched_task_group;
> +#else
> + return true;
> +#endif /* CONFIG_SCHED_AUTOGROUP */
> +}
> +
> +static void sched_change_group(struct task_struct *tsk)
> +{
> + tsk->sched_task_group = sched_get_task_group(tsk);
>
> #ifdef CONFIG_FAIR_GROUP_SCHED
> if (tsk->sched_class->task_change_group)
> @@ -10367,6 +10388,10 @@ void sched_move_task(struct task_struct *tsk)
> struct rq *rq;
>
> rq = task_rq_lock(tsk, &rf);
> +
> + if (!sched_task_group_changed(tsk))
> + goto unlock;
> +
> update_rq_clock(rq);
>
> running = task_current(rq, tsk);
> @@ -10391,6 +10416,7 @@ void sched_move_task(struct task_struct *tsk)
> resched_curr(rq);
> }
>
> +unlock:
> task_rq_unlock(rq, tsk, &rf);
> }
Would you mind terribly if I change it like so?
---
Subject: sched/core: Reduce cost of sched_move_task when config autogroup
From: wuchi <wuchi.zero@...il.com>
Date: Tue, 21 Mar 2023 14:44:59 +0800
From: wuchi <wuchi.zero@...il.com>
Some sched_move_task calls are useless because that
task_struct->sched_task_group maybe not changed (equals task_group
of cpu_cgroup) when system enable autogroup. So do some checks in
sched_move_task.
sched_move_task eg:
task A belongs to cpu_cgroup0 and autogroup0, it will always belong
to cpu_cgroup0 when do_exit. So there is no need to do {de|en}queue.
The call graph is as follow.
do_exit
sched_autogroup_exit_task
sched_move_task
dequeue_task
sched_change_group
A.sched_task_group = sched_get_task_group (=cpu_cgroup0)
enqueue_task
Performance results:
===========================
1. env
cpu: bogomips=4600.00
kernel: 6.3.0-rc3
cpu_cgroup: 6:cpu,cpuacct:/user.slice
2. cmds
do_exit script:
for i in {0..10000}; do
sleep 0 &
done
wait
Run the above script, then use the following bpftrace cmd to get
the cost of sched_move_task:
bpftrace -e 'k:sched_move_task { @ts[tid] = nsecs; }
kr:sched_move_task /@ts[tid]/
{ @ns += nsecs - @ts[tid]; delete(@ts[tid]); }'
3. cost time(ns):
without patch: 43528033
with patch: 18541416
diff:-24986617 -57.4%
As the result show, the patch will save 57.4% in the scenario.
Signed-off-by: wuchi <wuchi.zero@...il.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20230321064459.39421-1-wuchi.zero@gmail.com
---
kernel/sched/core.c | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10351,7 +10351,7 @@ void sched_release_group(struct task_gro
spin_unlock_irqrestore(&task_group_lock, flags);
}
-static void sched_change_group(struct task_struct *tsk)
+static struct task_group *sched_get_task_group(struct task_struct *tsk)
{
struct task_group *tg;
@@ -10363,7 +10363,13 @@ static void sched_change_group(struct ta
tg = container_of(task_css_check(tsk, cpu_cgrp_id, true),
struct task_group, css);
tg = autogroup_task_group(tsk, tg);
- tsk->sched_task_group = tg;
+
+ return tg;
+}
+
+static void sched_change_group(struct task_struct *tsk, struct task_group *group)
+{
+ tsk->sched_task_group = group;
#ifdef CONFIG_FAIR_GROUP_SCHED
if (tsk->sched_class->task_change_group)
@@ -10384,10 +10390,19 @@ void sched_move_task(struct task_struct
{
int queued, running, queue_flags =
DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK;
+ struct task_group *group;
struct rq_flags rf;
struct rq *rq;
rq = task_rq_lock(tsk, &rf);
+ /*
+ * Esp. with SCHED_AUTOGROUP enabled it is possible to get superfluous
+ * group changes.
+ */
+ group = sched_get_task_group(tsk);
+ if (group == tsk->sched_task_group)
+ goto unlock;
+
update_rq_clock(rq);
running = task_current(rq, tsk);
@@ -10398,7 +10413,7 @@ void sched_move_task(struct task_struct
if (running)
put_prev_task(rq, tsk);
- sched_change_group(tsk);
+ sched_change_group(tsk, group);
if (queued)
enqueue_task(rq, tsk, queue_flags);
@@ -10412,6 +10427,7 @@ void sched_move_task(struct task_struct
resched_curr(rq);
}
+unlock:
task_rq_unlock(rq, tsk, &rf);
}
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