linux-kernel - Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with sched_domain

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Date:	Wed, 11 May 2016 19:37:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org, clm@...com,
	mgalbraith@...e.de, tglx@...utronix.de, fweisbec@...il.com
Subject: Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with
 sched_domain_shared

On Wed, May 11, 2016 at 12:55:56PM +0100, Matt Fleming wrote:

> This breaks my POWER7 box which presumably doesn't have SD_SHARE_PKG_RESOURCES,

> index 978b3ef2d87e..d27153adee4d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7920,7 +7920,8 @@ static inline void set_cpu_sd_state_busy(void)
>  		goto unlock;
>  	sd->nohz_idle = 0;
>  
> -	atomic_inc(&sd->shared->nr_busy_cpus);
> +	if (sd->shared)
> +		atomic_inc(&sd->shared->nr_busy_cpus);
>  unlock:
>  	rcu_read_unlock();
>  }


Ah, no, the problem is that while it does have SHARE_PKG_RESOURCES (in
its SMT domain -- SMT threads share all cache after all), I failed to
connect the sched_domain_shared structure for it.

Does something like this also work?

---
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6389,10 +6389,6 @@ sd_init(struct sched_domain_topology_lev
 		sd->cache_nice_tries = 1;
 		sd->busy_idx = 2;
 
-		sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
-		atomic_inc(&sd->shared->ref);
-		atomic_set(&sd->shared->nr_busy_cpus, sd_weight);
-
 #ifdef CONFIG_NUMA
 	} else if (sd->flags & SD_NUMA) {
 		sd->cache_nice_tries = 2;
@@ -6414,6 +6410,16 @@ sd_init(struct sched_domain_topology_lev
 		sd->idle_idx = 1;
 	}
 
+	/*
+	 * For all levels sharing cache; connect a sched_domain_shared
+	 * instance.
+	 */
+	if (sd->flags & SH_SHARED_PKG_RESOURCES) {
+		sd->shared = *per_cpu_ptr(sdd->sds, sd_id);
+		atomic_inc(&sd->shared->ref);
+		atomic_set(&sd->shared->nr_busy_cpus, sd_weight);
+	}
+
 	sd->private = sdd;
 
 	return sd;