Re: [RFC PATCH v2] sched/fair: select idle cpu from idle cpumask in sched domain

[Li, Aubrey]

On 2020/9/16 19:00, Mel Gorman wrote:
> On Wed, Sep 16, 2020 at 12:31:03PM +0800, Aubrey Li wrote:
>> Added idle cpumask to track idle cpus in sched domain. When a CPU
>> enters idle, its corresponding bit in the idle cpumask will be set,
>> and when the CPU exits idle, its bit will be cleared.
>>
>> When a task wakes up to select an idle cpu, scanning idle cpumask
>> has low cost than scanning all the cpus in last level cache domain,
>> especially when the system is heavily loaded.
>>
>> The following benchmarks were tested on a x86 4 socket system with
>> 24 cores per socket and 2 hyperthreads per core, total 192 CPUs:
>>
> 
> This still appears to be tied to turning the tick off. An idle CPU
> available for computation does not necessarily have the tick turned off
> if it's for short periods of time. When nohz is disabled or a machine is
> active enough that CPUs are not disabling the tick, select_idle_cpu may
> fail to select an idle CPU and instead stack tasks on the old CPU.
> 
> The other subtlety is that select_idle_sibling() currently allows a
> SCHED_IDLE cpu to be used as a wakeup target. The CPU is not really
> idle as such, it's simply running a low priority task that is suitable
> for preemption. I suspect this patch breaks that.
> 
Thanks!

I shall post a v3 with performance data, I made a quick uperf testing and
found the benefit is still there. So I posted the patch here and looking
forward to your comments before I start the benchmarks.

Thanks,
-Aubrey

-----------------------------------------------------------------------
diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index fb11091129b3..43a641d26154 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -65,8 +65,21 @@ struct sched_domain_shared {
 	atomic_t	ref;
 	atomic_t	nr_busy_cpus;
 	int		has_idle_cores;
+	/*
+	 * Span of all idle CPUs in this domain.
+	 *
+	 * NOTE: this field is variable length. (Allocated dynamically
+	 * by attaching extra space to the end of the structure,
+	 * depending on how many CPUs the kernel has booted up with)
+	 */
+	unsigned long	idle_cpus_span[];
 };
 
+static inline struct cpumask *sds_idle_cpus(struct sched_domain_shared *sds)
+{
+	return to_cpumask(sds->idle_cpus_span);
+}
+
 struct sched_domain {
 	/* These fields must be setup */
 	struct sched_domain __rcu *parent;	/* top domain must be null terminated */
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6b3b59cc51d6..9a3c82645472 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6023,6 +6023,26 @@ void __update_idle_core(struct rq *rq)
 	rcu_read_unlock();
 }
 
+/*
+ * Update cpu idle state and record this information
+ * in sd_llc_shared->idle_cpus_span.
+ */
+void update_idle_cpumask(struct rq *rq)
+{
+	struct sched_domain *sd;
+	int cpu = cpu_of(rq);
+
+	rcu_read_lock();
+	sd = rcu_dereference(per_cpu(sd_llc, cpu));
+	if (!sd || !sd->shared)
+		goto unlock;
+	if (!available_idle_cpu(cpu) || !sched_idle_cpu(cpu))
+		goto unlock;
+	cpumask_set_cpu(cpu, sds_idle_cpus(sd->shared));
+unlock:
+	rcu_read_unlock();
+}
+
 /*
  * Scan the entire LLC domain for idle cores; this dynamically switches off if
  * there are no idle cores left in the system; tracked through
@@ -6136,7 +6156,12 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
 
 	time = cpu_clock(this);
 
-	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+	/*
+	 * sched_domain_shared is set only at shared cache level,
+	 * this works only because select_idle_cpu is called with
+	 * sd_llc.
+	 */
+	cpumask_and(cpus, sds_idle_cpus(sd->shared), p->cpus_ptr);
 
 	for_each_cpu_wrap(cpu, cpus, target) {
 		if (!--nr)
@@ -6712,6 +6737,10 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
 
 		if (want_affine)
 			current->recent_used_cpu = cpu;
+
+		sd = rcu_dereference(per_cpu(sd_llc, new_cpu));
+		if (sd && sd->shared)
+			cpumask_clear_cpu(new_cpu, sds_idle_cpus(sd->shared));
 	}
 	rcu_read_unlock();
 
@@ -10871,6 +10900,9 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
 		/* ensure bandwidth has been allocated on our new cfs_rq */
 		account_cfs_rq_runtime(cfs_rq, 0);
 	}
+	/* Update idle cpumask if task has idle policy */
+	if (unlikely(task_has_idle_policy(p)))
+		update_idle_cpumask(rq);
 }
 
 void init_cfs_rq(struct cfs_rq *cfs_rq)
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 1ae95b9150d3..876dfdfe35bb 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -405,6 +405,7 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
 static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
 {
 	update_idle_core(rq);
+	update_idle_cpumask(rq);
 	schedstat_inc(rq->sched_goidle);
 }
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c82857e2e288..7a3355f61bcf 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1069,6 +1069,7 @@ static inline void update_idle_core(struct rq *rq)
 #else
 static inline void update_idle_core(struct rq *rq) { }
 #endif
+void update_idle_cpumask(struct rq *rq);
 
 DECLARE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
 
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 9079d865a935..f14a6ef4de57 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1407,6 +1407,7 @@ sd_init(struct sched_domain_topology_level *tl,
 		sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
 		atomic_inc(&sd->shared->ref);
 		atomic_set(&sd->shared->nr_busy_cpus, sd_weight);
+		cpumask_copy(sds_idle_cpus(sd->shared), sched_domain_span(sd));
 	}
 
 	sd->private = sdd;
@@ -1769,7 +1770,7 @@ static int __sdt_alloc(const struct cpumask *cpu_map)
 
 			*per_cpu_ptr(sdd->sd, j) = sd;
 
-			sds = kzalloc_node(sizeof(struct sched_domain_shared),
+			sds = kzalloc_node(sizeof(struct sched_domain_shared) + cpumask_size(),
 					GFP_KERNEL, cpu_to_node(j));
 			if (!sds)
 				return -ENOMEM;