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Date: Wed, 18 Aug 2021 00:45:17 +0800
From: Tao Zhou <tao.zhou@...ux.dev>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de,
joel@...lfernandes.org, chris.hyser@...cle.com, joshdon@...gle.com,
mingo@...nel.org, vincent.guittot@...aro.org,
valentin.schneider@....com, mgorman@...e.de, taozhou@...ux.dev
Subject: Re: [PATCH] sched/core: An optimization of pick_next_task() not sure
Hi Peter,
On Mon, Aug 16, 2021 at 08:52:39PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 16, 2021 at 11:44:01PM +0800, Tao Zhou wrote:
> > When find a new candidate max, wipe the stale and start over.
> > Goto again: and use the new max to loop to pick the the task.
> >
> > Here first want to get the max of the core and use this new
> > max to loop once to pick the task on each thread.
> >
> > Not sure this is an optimization and just stop here a little
> > and move on..
> >
>
> Did you find this retry was an issue on your workload? Or was this from
> reading the source?
Thank you for your reply. Sorry for my late reply.
This was from reading the source..
>
> > ---
> > kernel/sched/core.c | 52 +++++++++++++++++----------------------------
> > 1 file changed, 20 insertions(+), 32 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 20ffcc044134..bddcd328df96 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -5403,7 +5403,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> > const struct sched_class *class;
> > const struct cpumask *smt_mask;
> > bool fi_before = false;
> > - int i, j, cpu, occ = 0;
> > + int i, cpu, occ = 0;
> > bool need_sync;
> >
> > if (!sched_core_enabled(rq))
> > @@ -5508,11 +5508,27 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> > * order.
> > */
> > for_each_class(class) {
> > -again:
> > + struct rq *rq_i;
> > + struct task_struct *p;
> > +
> > for_each_cpu_wrap(i, smt_mask, cpu) {
> > - struct rq *rq_i = cpu_rq(i);
> > - struct task_struct *p;
> > + rq_i = cpu_rq(i);
> > + p = pick_task(rq_i, class, max, fi_before);
> > + /*
> > + * If this new candidate is of higher priority than the
> > + * previous; and they're incompatible; pick_task makes
> > + * sure that p's priority is more than max if it doesn't
> > + * match max's cookie. Update max.
> > + *
> > + * NOTE: this is a linear max-filter and is thus bounded
> > + * in execution time.
> > + */
> > + if (!max || !cookie_match(max, p))
> > + max = p;
> > + }
> >
> > + for_each_cpu_wrap(i, smt_mask, cpu) {
> > + rq_i = cpu_rq(i);
> > if (rq_i->core_pick)
> > continue;
> >
>
> This now calls pick_task() twice for each CPU, which seems unfortunate;
> perhaps add q->core_temp storage to cache that result. Also, since the
> first iteration is now explicitly about the max filter, perhaps we
> shouuld move that part of pick_task() into the loop and simplify things
> further?
Here is my ugly patch below..
Not compiled..
>From b3de16fb6f3e6cd2a8a9f7a579e80df74fb2d865 Mon Sep 17 00:00:00 2001
From: Tao Zhou <tao.zhou@...ux.dev>
Date: Wed, 18 Aug 2021 00:07:38 +0800
Subject: [PATCH] optimize pick_next_task()
---
kernel/sched/core.c | 71 +++++++++++++++++++++++++++++++++-----------
kernel/sched/sched.h | 1 +
2 files changed, 54 insertions(+), 18 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 20ffcc044134..c2a403bacf99 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5380,18 +5380,32 @@ pick_task(struct rq *rq, const struct sched_class *class, struct task_struct *ma
if (cookie_equals(class_pick, cookie))
return class_pick;
- cookie_pick = sched_core_find(rq, cookie);
+ return class_pick;
+}
- /*
- * If class > max && class > cookie, it is the highest priority task on
- * the core (so far) and it must be selected, otherwise we must go with
- * the cookie pick in order to satisfy the constraint.
- */
- if (prio_less(cookie_pick, class_pick, in_fi) &&
- (!max || prio_less(max, class_pick, in_fi)))
- return class_pick;
+static task_struct *
+filter_max_prio(struct rq *rq, struct task_struct *class_pick,
+ struct task_struct **cookie_pick, struct task_struct *max,
+ bool in_fi)
+{
+ unsigned long cookie = rq->core->core_cookie;
- return cookie_pick;
+ *cookie_pick = NULL;
+ if (cookie && !cookie_equals(class_pick, cookie)) {
+ *cookie_pick = sched_core_find(rq, cookie);
+ /*
+ * If class > max && class > cookie, it is the
+ * highest priority task on the core (so far)
+ * and it must be selected, otherwise we must
+ * go with the cookie pick in order to satisfy
+ * the constraint.
+ */
+ if (prio_less(cookie_pick, class_pick, in_fi) &&
+ (!max || prio_less(max, class_pick, in_fi)))
+ return class_pick;
+ }
+
+ return NULL;
}
extern void task_vruntime_update(struct rq *rq, struct task_struct *p, bool in_fi);
@@ -5508,24 +5522,44 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
* order.
*/
for_each_class(class) {
-again:
+ struct task_struct *class_pick, *cookie_pick;
+ struct rq *rq_i;
+
+ for_each_cpu_wrap(i, smt_mask, cpu) {
+ rq_i = cpu_rq(i);
+ class_pick = pick_task(rq_i, class, max, fi_before);
+ rq_i->core_temp = class_pick;
+ /*
+ * This sibling doesn't yet have a suitable task to
+ * run.
+ */
+ if (!class_pick)
+ continue;
+
+ if (filter_max_prio(rq_i, class_pick, &cookie_pick, max, fi_before))
+ max = class_pick;
+ }
+
for_each_cpu_wrap(i, smt_mask, cpu) {
- struct rq *rq_i = cpu_rq(i);
struct task_struct *p;
+ rq_i = cpu_rq(i);
if (rq_i->core_pick)
continue;
/*
- * If this sibling doesn't yet have a suitable task to
- * run; ask for the most eligible task, given the
- * highest priority task already selected for this
- * core.
+ * This sibling doesn't yet have a suitable task to
+ * run.
*/
- p = pick_task(rq_i, class, max, fi_before);
- if (!p)
+ if (!rq_i->core_temp)
continue;
+ p = class_pick = rq_i->core_temp;
+ if (!filter_max_prio(rq_i, class_pick, &cookie_pick, max, fi_before)) {
+ if (cookie_pick)
+ p = cookie_pick;
+ }
+
if (!is_task_rq_idle(p))
occ++;
@@ -9024,6 +9058,7 @@ void __init sched_init(void)
#ifdef CONFIG_SCHED_CORE
rq->core = NULL;
rq->core_pick = NULL;
+ rq->core_temp = NULL;
rq->core_enabled = 0;
rq->core_tree = RB_ROOT;
rq->core_forceidle = false;
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 14a41a243f7b..2b21a3846b8e 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1089,6 +1089,7 @@ struct rq {
/* per rq */
struct rq *core;
struct task_struct *core_pick;
+ struct task_struct *core_temp;
unsigned int core_enabled;
unsigned int core_sched_seq;
struct rb_root core_tree;
--
2.31.1
Thanks,
Tao
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