Re: [PATCH 5/5] sched/fair: Skip LLC nohz logic for asymmetric systems

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 17, 2019 at 03:34:11PM +0000, Valentin Schneider wrote:
> The LLC nohz condition will become true as soon as >=2 CPUs in a
> single LLC domain are busy. On big.LITTLE systems, this translates to
> two or more CPUs of a "cluster" (big or LITTLE) being busy.
> 
> Issuing a nohz kick in these conditions isn't desired for asymmetric
> systems, as if the busy CPUs can provide enough compute capacity to
> the running tasks, then we can leave the nohz CPUs in peace.
> 
> Skip the LLC nohz condition for asymmetric systems, and rely on
> nr_running & capacity checks to trigger nohz kicks when the system
> actually needs them.
> 
> Suggested-by: Morten Rasmussen <morten.rasmussen@arm.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
>  kernel/sched/fair.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 291dfdb0183f..ff27bf56e8d4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9545,6 +9545,17 @@ static void nohz_balancer_kick(struct rq *rq)
>  	}
>  
>  	rcu_read_lock();
> +
> +	if (static_branch_unlikely(&sched_asym_cpucapacity))
> +		/*
> +		 * For asymmetric systems, we do not want to nicely balance
> +		 * cache use, instead we want to embrace asymmetry and only
> +		 * ensure tasks have enough CPU capacity.
> +		 *
> +		 * Skip the LLC logic because it's not relevant in that case.
> +		 */
> +		goto check_capacity;
> +
>  	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
>  	if (sds) {
>  		/*

Since (before this) the actual order of the various tests doesn't
matter, it's a logical cascade of conditions for which to KICK_MASK.

We can easily reorder and short-circuit the cascase like so, no?

The only concern is if sd_llc_shared < sd_asym_capacity; in which case
we just lost a balance opportunity. Not sure how to best retain that
though.

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9568,25 +9568,6 @@ static void nohz_balancer_kick(struct rq
 	}
 
 	rcu_read_lock();
-	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
-	if (sds) {
-		/*
-		 * If there is an imbalance between LLC domains (IOW we could
-		 * increase the overall cache use), we need some less-loaded LLC
-		 * domain to pull some load. Likewise, we may need to spread
-		 * load within the current LLC domain (e.g. packed SMT cores but
-		 * other CPUs are idle). We can't really know from here how busy
-		 * the others are - so just get a nohz balance going if it looks
-		 * like this LLC domain has tasks we could move.
-		 */
-		nr_busy = atomic_read(&sds->nr_busy_cpus);
-		if (nr_busy > 1) {
-			flags = NOHZ_KICK_MASK;
-			goto unlock;
-		}
-
-	}
-
 	sd = rcu_dereference(rq->sd);
 	if (sd) {
 		/*
@@ -9600,6 +9581,20 @@ static void nohz_balancer_kick(struct rq
 		}
 	}
 
+	sd = rcu_dereference(per_cpu(sd_asym_packing, cpu));
+	if (sd) {
+		/*
+		 * When ASYM_PACKING; see if there's a more preferred CPU going
+		 * idle; in which case, kick the ILB to move tasks around.
+		 */
+		for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask) {
+			if (sched_asym_prefer(i, cpu)) {
+				flags = NOHZ_KICK_MASK;
+				goto unlock;
+			}
+		}
+	}
+
 	sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu));
 	if (sd) {
 		/*
@@ -9610,21 +9605,36 @@ static void nohz_balancer_kick(struct rq
 			flags = NOHZ_KICK_MASK;
 			goto unlock;
 		}
+
+		/*
+		 * For asymmetric systems, we do not want to nicely balance
+		 * cache use, instead we want to embrace asymmetry and only
+		 * ensure tasks have enough CPU capacity.
+		 *
+		 * Skip the LLC logic because it's not relevant in that case.
+		 */
+		goto unlock;
 	}
 
-	sd = rcu_dereference(per_cpu(sd_asym_packing, cpu));
-	if (sd) {
+	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
+	if (sds) {
 		/*
-		 * When ASYM_PACKING; see if there's a more preferred CPU going
-		 * idle; in which case, kick the ILB to move tasks around.
+		 * If there is an imbalance between LLC domains (IOW we could
+		 * increase the overall cache use), we need some less-loaded LLC
+		 * domain to pull some load. Likewise, we may need to spread
+		 * load within the current LLC domain (e.g. packed SMT cores but
+		 * other CPUs are idle). We can't really know from here how busy
+		 * the others are - so just get a nohz balance going if it looks
+		 * like this LLC domain has tasks we could move.
 		 */
-		for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask) {
-			if (sched_asym_prefer(i, cpu)) {
-				flags = NOHZ_KICK_MASK;
-				goto unlock;
-			}
+		nr_busy = atomic_read(&sds->nr_busy_cpus);
+		if (nr_busy > 1) {
+			flags = NOHZ_KICK_MASK;
+			goto unlock;
 		}
+
 	}
+
 unlock:
 	rcu_read_unlock();
 out: