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Subject: Re: [PATCH -tip 14/32] sched: migration changes for core scheduling
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Balbir Singh <bsingharora@gmail.com>,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vineeth Pillai <viremana@linux.microsoft.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>, tglx@linutronix.de,
        linux-kernel@vger.kernel.org, mingo@kernel.org,
        torvalds@linux-foundation.org, fweisbec@gmail.com,
        keescook@chromium.org, kerrnel@google.com,
        Phil Auld <pauld@redhat.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>, vineeth@bitbyteword.org,
        Chen Yu <yu.c.chen@intel.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Agata Gruza <agata.gruza@intel.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@intel.com>,
        graf@amazon.com, konrad.wilk@oracle.com, dfaggioli@suse.com,
        pjt@google.com, rostedt@goodmis.org, derkling@google.com,
        benbjiang@tencent.com,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        James.Bottomley@hansenpartnership.com, OWeisse@umich.edu,
        Dhaval Giani <dhaval.giani@oracle.com>,
        Junaid Shahid <junaids@google.com>, jsbarnes@google.com,
        chris.hyser@oracle.com, Ben Segall <bsegall@google.com>,
        Josh Don <joshdon@google.com>, Hao Luo <haoluo@google.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Aubrey Li <aubrey.li@intel.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Tim Chen <tim.c.chen@intel.com>
References: <20201117232003.3580179-1-joel@joelfernandes.org>
 <20201117232003.3580179-15-joel@joelfernandes.org>
 <20201122235456.GF110669@balbir-desktop>
 <0b2514ef-6cc3-c1a3-280b-5d9062c80a31@linux.intel.com>
 <20201124154237.GZ3021@hirez.programming.kicks-ass.net>
From:   "Li, Aubrey" <aubrey.li@linux.intel.com>
Message-ID: <d541b70c-c65f-5bf6-5e71-0b9b35457fae@linux.intel.com>
Date:   Wed, 25 Nov 2020 11:12:53 +0800
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On 2020/11/24 23:42, Peter Zijlstra wrote:
> On Mon, Nov 23, 2020 at 12:36:10PM +0800, Li, Aubrey wrote:
>>>> +#ifdef CONFIG_SCHED_CORE
>>>> +		/*
>>>> +		 * Skip this cpu if source task's cookie does not match
>>>> +		 * with CPU's core cookie.
>>>> +		 */
>>>> +		if (!sched_core_cookie_match(cpu_rq(cpu), env->p))
>>>> +			continue;
>>>> +#endif
>>>> +
>>>
>>> Any reason this is under an #ifdef? In sched_core_cookie_match() won't
>>> the check for sched_core_enabled() do the right thing even when
>>> CONFIG_SCHED_CORE is not enabed?> 
>> Yes, sched_core_enabled works properly when CONFIG_SCHED_CORE is not
>> enabled. But when CONFIG_SCHED_CORE is not enabled, it does not make
>> sense to leave a core scheduler specific function here even at compile
>> time. Also, for the cases in hot path, this saves CPU cycles to avoid
>> a judgment.
> 
> No, that's nonsense. If it works, remove the #ifdef. Less (#ifdef) is
> more.
> 

Okay, I pasted the refined patch here.
@Joel, please let me know if you want me to send it in a separated thread.

Thanks,
-Aubrey
======================================================================
From 18e4f4592c2a159fcbae637f3a422e37ad24cb5a Mon Sep 17 00:00:00 2001
From: Aubrey Li <aubrey.li@linux.intel.com>
Date: Wed, 25 Nov 2020 02:43:46 +0000
Subject: [PATCH 14/33] sched: migration changes for core scheduling

 - Don't migrate if there is a cookie mismatch
     Load balance tries to move task from busiest CPU to the
     destination CPU. When core scheduling is enabled, if the
     task's cookie does not match with the destination CPU's
     core cookie, this task will be skipped by this CPU. This
     mitigates the forced idle time on the destination CPU.

 - Select cookie matched idle CPU
     In the fast path of task wakeup, select the first cookie matched
     idle CPU instead of the first idle CPU.

 - Find cookie matched idlest CPU
     In the slow path of task wakeup, find the idlest CPU whose core
     cookie matches with task's cookie

 - Don't migrate task if cookie not match
     For the NUMA load balance, don't migrate task to the CPU whose
     core cookie does not match with task's cookie

Tested-by: Julien Desfossez <jdesfossez@digitalocean.com>
Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Vineeth Remanan Pillai <viremana@linux.microsoft.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/sched/fair.c  | 58 ++++++++++++++++++++++++++++++++++++++++----
 kernel/sched/sched.h | 33 +++++++++++++++++++++++++
 2 files changed, 86 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index de82f88ba98c..7eea5da6685a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1921,6 +1921,13 @@ static void task_numa_find_cpu(struct task_numa_env *env,
 		if (!cpumask_test_cpu(cpu, env->p->cpus_ptr))
 			continue;
 
+		/*
+		 * Skip this cpu if source task's cookie does not match
+		 * with CPU's core cookie.
+		 */
+		if (!sched_core_cookie_match(cpu_rq(cpu), env->p))
+			continue;
+
 		env->dst_cpu = cpu;
 		if (task_numa_compare(env, taskimp, groupimp, maymove))
 			break;
@@ -5867,11 +5874,15 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this
 
 	/* Traverse only the allowed CPUs */
 	for_each_cpu_and(i, sched_group_span(group), p->cpus_ptr) {
+		struct rq *rq = cpu_rq(i);
+
+		if (!sched_core_cookie_match(rq, p))
+			continue;
+
 		if (sched_idle_cpu(i))
 			return i;
 
 		if (available_idle_cpu(i)) {
-			struct rq *rq = cpu_rq(i);
 			struct cpuidle_state *idle = idle_get_state(rq);
 			if (idle && idle->exit_latency < min_exit_latency) {
 				/*
@@ -6129,8 +6140,18 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 	for_each_cpu_wrap(cpu, cpus, target) {
 		if (!--nr)
 			return -1;
-		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
-			break;
+
+		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu)) {
+#ifdef CONFIG_SCHED_CORE
+			/*
+			 * If Core Scheduling is enabled, select this cpu
+			 * only if the process cookie matches core cookie.
+			 */
+			if (sched_core_enabled(cpu_rq(cpu)) &&
+			    p->core_cookie == cpu_rq(cpu)->core->core_cookie)
+#endif
+				break;
+		}
 	}
 
 	time = cpu_clock(this) - time;
@@ -7530,8 +7551,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 	 * We do not migrate tasks that are:
 	 * 1) throttled_lb_pair, or
 	 * 2) cannot be migrated to this CPU due to cpus_ptr, or
-	 * 3) running (obviously), or
-	 * 4) are cache-hot on their current CPU.
+	 * 3) task's cookie does not match with this CPU's core cookie
+	 * 4) running (obviously), or
+	 * 5) are cache-hot on their current CPU.
 	 */
 	if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
 		return 0;
@@ -7566,6 +7588,13 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 		return 0;
 	}
 
+	/*
+	 * Don't migrate task if the task's cookie does not match
+	 * with the destination CPU's core cookie.
+	 */
+	if (!sched_core_cookie_match(cpu_rq(env->dst_cpu), p))
+		return 0;
+
 	/* Record that we found atleast one task that could run on dst_cpu */
 	env->flags &= ~LBF_ALL_PINNED;
 
@@ -8792,6 +8821,25 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
 					p->cpus_ptr))
 			continue;
 
+#ifdef CONFIG_SCHED_CORE
+		if (sched_core_enabled(cpu_rq(this_cpu))) {
+			int i = 0;
+			bool cookie_match = false;
+
+			for_each_cpu(i, sched_group_span(group)) {
+				struct rq *rq = cpu_rq(i);
+
+				if (sched_core_cookie_match(rq, p)) {
+					cookie_match = true;
+					break;
+				}
+			}
+			/* Skip over this group if no cookie matched */
+			if (!cookie_match)
+				continue;
+		}
+#endif
+
 		local_group = cpumask_test_cpu(this_cpu,
 					       sched_group_span(group));
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e72942a9ee11..05b93787fe62 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1135,6 +1135,35 @@ static inline raw_spinlock_t *rq_lockp(struct rq *rq)
 
 bool cfs_prio_less(struct task_struct *a, struct task_struct *b);
 
+/*
+ * Helper to check if the CPU's core cookie matches with the task's cookie
+ * when core scheduling is enabled.
+ * A special case is that the task's cookie always matches with CPU's core
+ * cookie if the CPU is in an idle core.
+ */
+static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
+{
+	bool idle_core = true;
+	int cpu;
+
+	/* Ignore cookie match if core scheduler is not enabled on the CPU. */
+	if (!sched_core_enabled(rq))
+		return true;
+
+	for_each_cpu(cpu, cpu_smt_mask(cpu_of(rq))) {
+		if (!available_idle_cpu(cpu)) {
+			idle_core = false;
+			break;
+		}
+	}
+
+	/*
+	 * A CPU in an idle core is always the best choice for tasks with
+	 * cookies.
+	 */
+	return idle_core || rq->core->core_cookie == p->core_cookie;
+}
+
 extern void queue_core_balance(struct rq *rq);
 
 #else /* !CONFIG_SCHED_CORE */
@@ -1153,6 +1182,10 @@ static inline void queue_core_balance(struct rq *rq)
 {
 }
 
+static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
+{
+	return true;
+}
 #endif /* CONFIG_SCHED_CORE */
 
 #ifdef CONFIG_SCHED_SMT
-- 
2.17.1