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Date:   Fri, 22 Mar 2019 14:34:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Julien Desfossez <jdesfossez@...italocean.com>
Cc:     mingo@...nel.org, tglx@...utronix.de, pjt@...gle.com,
        tim.c.chen@...ux.intel.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, subhra.mazumdar@...cle.com,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Vineeth Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>
Subject: Re: [RFC][PATCH 03/16] sched: Wrap rq::lock access

On Thu, Mar 21, 2019 at 05:20:17PM -0400, Julien Desfossez wrote:
> On further investigation, we could see that the contention is mostly in the
> way rq locks are taken. With this patchset, we lock the whole core if
> cpu.tag is set for at least one cgroup. Due to this, __schedule() is more or
> less serialized for the core and that attributes to the performance loss
> that we are seeing. We also saw that newidle_balance() takes considerably
> long time in load_balance() due to the rq spinlock contention. Do you think
> it would help if the core-wide locking was only performed when absolutely
> needed ?

Something like that could be done, but then you end up with 2 locks,
something which I was hoping to avoid.

Basically you keep rq->lock as it exists today, but add something like
rq->core->core_lock, you then have to take that second lock (nested
under rq->lock) for every scheduling action involving a tagged task.

It makes things complicatd though; because now my head hurts thikning
about pick_next_task().

(this can obviously do away with the whole rq->lock wrappery)

Also, completely untested..

---
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -146,6 +146,8 @@ void sched_core_enqueue(struct rq *rq, s
 	if (!p->core_cookie)
 		return;
 
+	raw_spin_lock(&rq->core->core_lock);
+
 	node = &rq->core_tree.rb_node;
 	parent = *node;
 
@@ -161,6 +163,8 @@ void sched_core_enqueue(struct rq *rq, s
 
 	rb_link_node(&p->core_node, parent, node);
 	rb_insert_color(&p->core_node, &rq->core_tree);
+
+	raw_spin_unlock(&rq->core->core_lock);
 }
 
 void sched_core_dequeue(struct rq *rq, struct task_struct *p)
@@ -170,7 +174,9 @@ void sched_core_dequeue(struct rq *rq, s
 	if (!p->core_cookie)
 		return;
 
+	raw_spin_lock(&rq->core->core_lock);
 	rb_erase(&p->core_node, &rq->core_tree);
+	raw_spin_unlock(&rq->core->core_lock);
 }
 
 /*
@@ -181,6 +187,8 @@ struct task_struct *sched_core_find(stru
 	struct rb_node *node = rq->core_tree.rb_node;
 	struct task_struct *node_task, *match;
 
+	lockdep_assert_held(&rq->core->core_lock);
+
 	/*
 	 * The idle task always matches any cookie!
 	 */
@@ -206,6 +214,8 @@ struct task_struct *sched_core_next(stru
 {
 	struct rb_node *node = &p->core_node;
 
+	lockdep_assert_held(&rq->core->core_lock);
+
 	node = rb_next(node);
 	if (!node)
 		return NULL;
@@ -3685,6 +3695,8 @@ pick_next_task(struct rq *rq, struct tas
 	 * If there were no {en,de}queues since we picked (IOW, the task
 	 * pointers are all still valid), and we haven't scheduled the last
 	 * pick yet, do so now.
+	 *
+	 * XXX probably OK without ->core_lock
 	 */
 	if (rq->core->core_pick_seq == rq->core->core_task_seq &&
 	    rq->core->core_pick_seq != rq->core_sched_seq) {
@@ -3710,6 +3722,20 @@ pick_next_task(struct rq *rq, struct tas
 	if (!rq->nr_running)
 		newidle_balance(rq, rf);
 
+	if (!rq->core->core_cookie) {
+		for_each_class(class) {
+			next = pick_task(rq, class, NULL);
+			if (next)
+				break;
+		}
+
+		if (!next->core_cookie) {
+			set_next_task(rq, next);
+			return next;
+		}
+	}
+
+	raw_spin_lock(&rq->core->core_lock);
 	cpu = cpu_of(rq);
 	smt_mask = cpu_smt_mask(cpu);
 
@@ -3849,6 +3875,7 @@ next_class:;
 	trace_printk("picked: %s/%d %lx\n", next->comm, next->pid, next->core_cookie);
 
 done:
+	raw_spin_unlock(&rq->core->core_lock);
 	set_next_task(rq, next);
 	return next;
 }
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -966,6 +966,7 @@ struct rq {
 	struct rb_root		core_tree;
 
 	/* shared state */
+	raw_spinlock_t		core_lock;
 	unsigned int		core_task_seq;
 	unsigned int		core_pick_seq;
 	unsigned long		core_cookie;
@@ -1007,9 +1008,6 @@ static inline bool sched_core_enabled(st
 
 static inline raw_spinlock_t *rq_lockp(struct rq *rq)
 {
-	if (sched_core_enabled(rq))
-		return &rq->core->__lock;
-
 	return &rq->__lock;
 }
 

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