Re: [patch 00/17] CFS Bandwidth Control v7.1

[Peter Zijlstra]

On Thu, 2011-07-07 at 16:38 +0200, Peter Zijlstra wrote:
> On Thu, 2011-07-07 at 13:23 +0200, Ingo Molnar wrote:
> > 
> > The +1.5% increase in vanilla kernel context switching performance is 
> > unfortunate - where does that overhead come from?
> 
> Looking at the asm output, I think its partly because things like:
> 
> @@ -602,6 +618,8 @@ static void update_curr(struct cfs_rq *c
>                 cpuacct_charge(curtask, delta_exec);
>                 account_group_exec_runtime(curtask, delta_exec);
>         }
> +
> +       account_cfs_rq_runtime(cfs_rq, delta_exec);
>  }
> 
> 
> +static void account_cfs_rq_runtime(struct cfs_rq *cfs_rq,
> +               unsigned long delta_exec)
> +{
> +       if (!cfs_rq->runtime_enabled)
> +               return;
> +
> +       cfs_rq->runtime_remaining -= delta_exec;
> +       if (cfs_rq->runtime_remaining > 0)
> +               return;
> +
> +       assign_cfs_rq_runtime(cfs_rq);
> +}
> 
> generate a call, only to then take the first branch out, marking that
> function __always_inline would cure the call problem. Going beyond that
> would be using static_branch() to track if there is any bandwidth
> tracking required at all.

Right, so that cfs_rq->runtime_enabled is almost a guaranteed cacheline
miss as well, its at the tail end of cfs_rq, then again, the smp-load
update will want to touch that same cacheline so its not a complete
waste of time.

The other big addition to all the fast paths are the various throttled
checks, those do miss a complete new cacheline.. adding a
static_branch() to that might make sense.

compile tested only..

---
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -71,6 +71,7 @@
 #include <linux/ctype.h>
 #include <linux/ftrace.h>
 #include <linux/slab.h>
+#include <linux/jump_label.h>
 
 #include <asm/tlb.h>
 #include <asm/irq_regs.h>
@@ -297,6 +298,7 @@ struct task_group {
 	struct autogroup *autogroup;
 #endif
 
+	int runtime_enabled;
 	struct cfs_bandwidth cfs_bandwidth;
 };
 
@@ -410,6 +412,8 @@ struct cfs_rq {
 };
 
 #ifdef CONFIG_CFS_BANDWIDTH
+static struct jump_label_key cfs_bandwidth_enabled;
+
 static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
 {
 	return &tg->cfs_bandwidth;
@@ -9075,6 +9079,15 @@ static int tg_set_cfs_bandwidth(struct t
 			unthrottle_cfs_rq(cfs_rq);
 		raw_spin_unlock_irq(&rq->lock);
 	}
+
+	if (runtime_enabled && !tg->runtime_enabled)
+		jump_label_inc(&cfs_bandwidth_enabled);
+
+	if (!runtime_enabled && tg->runtime_enabled)
+		jump_label_dec(&cfs_bandwidth_enabled);
+
+	tg->runtime_enabled = runtime_enabled;
+
 out_unlock:
 	mutex_unlock(&cfs_constraints_mutex);
 
Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c
+++ linux-2.6/kernel/sched_fair.c
@@ -1410,10 +1410,10 @@ static void expire_cfs_rq_runtime(struct
 	}
 }
 
-static void account_cfs_rq_runtime(struct cfs_rq *cfs_rq,
+static __always_inline void account_cfs_rq_runtime(struct cfs_rq *cfs_rq,
 		unsigned long delta_exec)
 {
-	if (!cfs_rq->runtime_enabled)
+	if (!static_branch(&cfs_bandwidth_enabled) || !cfs_rq->runtime_enabled)
 		return;
 
 	/* dock delta_exec before expiring quota (as it could span periods) */
@@ -1433,13 +1433,13 @@ static void account_cfs_rq_runtime(struc
 
 static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
 {
-	return cfs_rq->throttled;
+	return static_branch(&cfs_bandwidth_enabled) && cfs_rq->throttled;
 }
 
 /* check whether cfs_rq, or any parent, is throttled */
 static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
 {
-	return cfs_rq->throttle_count;
+	return static_branch(&cfs_bandwidth_enabled) && cfs_rq->throttle_count;
 }
 
 /*

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