Re: [PATCH v4 19/20] Optimize out cpu_clears

[Steven Rostedt]

On Tue, Nov 20, 2007 at 08:01:13PM -0500, Steven Rostedt wrote:
>  
>  static DEFINE_PER_CPU(cpumask_t, local_cpu_mask);
> -static DEFINE_PER_CPU(cpumask_t, valid_cpu_mask);
>  
>  static int find_lowest_cpus(struct task_struct *task, cpumask_t *lowest_mask)
>  {
> -	int       cpu;
> -	cpumask_t *valid_mask = &__get_cpu_var(valid_cpu_mask);
>  	int       lowest_prio = -1;
> +	int       lowest_cpu  = 0;
>  	int       count       = 0;
> +	int       cpu;
>  
> -	cpus_clear(*lowest_mask);
> -	cpus_and(*valid_mask, cpu_online_map, task->cpus_allowed);
> +	cpus_and(*lowest_mask, cpu_online_map, task->cpus_allowed);
>  
>  	/*
>  	 * Scan each rq for the lowest prio.
>  	 */
> -	for_each_cpu_mask(cpu, *valid_mask) {
> +	for_each_cpu_mask(cpu, *lowest_mask) {
>  		struct rq *rq = cpu_rq(cpu);
>  
>  		/* We look for lowest RT prio or non-rt CPU */
> @@ -325,13 +323,30 @@ static int find_lowest_cpus(struct task_
>  			if (rq->rt.highest_prio > lowest_prio) {
>  				/* new low - clear old data */
>  				lowest_prio = rq->rt.highest_prio;
> -				if (count) {
> -					cpus_clear(*lowest_mask);
> -					count = 0;
> -				}

Gregory Haskins pointed out to me that this logic is slightly wrong. I
originally wrote this patch before adding his "count" patch optimization.
I did not take into account that on finding a non RT queue, we may leave
on some extra bits because the clear_cpus is not preformed if count is
zero. And count gets set to zero here. Which means that we don't clean
up.

The fix is to check for lowest_cpus > 0 instead of count on finding an
non-RT runqueue. This lets us know that we need to clear the mask.
Otherwise, if lowest_cpus == 0, then we can return the mask untouched.
The proper bit would already be set, and the return of 1 will have
the rest of the algorithm use the first bit.

Below is the updated patch. The full series is at:

  http://rostedt.homelinux.com/rt/rt-balance-patches-v5.tar.bz2


> +				lowest_cpu = cpu;
> +				count = 0;
>  			}
>  			cpu_set(rq->cpu, *lowest_mask);
>  			count++;



From: Steven Rostedt <srostedt@xxxxxxxxxx>

This patch removes several cpumask operations by keeping track
of the first of the CPUS that is of the lowest priority. When
the search for the lowest priority runqueue is completed, all
the bits up to the first CPU with the lowest priority runqueue
is cleared.

Signed-off-by: Steven Rostedt <srostedt@xxxxxxxxxx>

---
 kernel/sched_rt.c |   48 ++++++++++++++++++++++++++++++++++++------------
 1 file changed, 36 insertions(+), 12 deletions(-)

Index: linux-compile.git/kernel/sched_rt.c
===================================================================
--- linux-compile.git.orig/kernel/sched_rt.c	2007-11-20 19:53:15.000000000 -0500
+++ linux-compile.git/kernel/sched_rt.c	2007-11-20 20:35:04.000000000 -0500
@@ -293,29 +293,36 @@ static struct task_struct *pick_next_hig
 }
 
 static DEFINE_PER_CPU(cpumask_t, local_cpu_mask);
-static DEFINE_PER_CPU(cpumask_t, valid_cpu_mask);
 
 static int find_lowest_cpus(struct task_struct *task, cpumask_t *lowest_mask)
 {
-	int       cpu;
-	cpumask_t *valid_mask = &__get_cpu_var(valid_cpu_mask);
 	int       lowest_prio = -1;
+	int       lowest_cpu  = 0;
 	int       count       = 0;
+	int       cpu;
 
-	cpus_clear(*lowest_mask);
-	cpus_and(*valid_mask, cpu_online_map, task->cpus_allowed);
+	cpus_and(*lowest_mask, cpu_online_map, task->cpus_allowed);
 
 	/*
 	 * Scan each rq for the lowest prio.
 	 */
-	for_each_cpu_mask(cpu, *valid_mask) {
+	for_each_cpu_mask(cpu, *lowest_mask) {
 		struct rq *rq = cpu_rq(cpu);
 
 		/* We look for lowest RT prio or non-rt CPU */
 		if (rq->rt.highest_prio >= MAX_RT_PRIO) {
-			if (count)
+			/*
+			 * if we already found a low RT queue
+			 * and now we found this non-rt queue
+			 * clear the mask and set our bit.
+			 * Otherwise just return the queue as is
+			 * and the count==1 will cause the algorithm
+			 * to use the first bit found.
+			 */
+			if (lowest_cpu) {
 				cpus_clear(*lowest_mask);
-			cpu_set(rq->cpu, *lowest_mask);
+				cpu_set(rq->cpu, *lowest_mask);
+			}
 			return 1;
 		}
 
@@ -325,13 +332,30 @@ static int find_lowest_cpus(struct task_
 			if (rq->rt.highest_prio > lowest_prio) {
 				/* new low - clear old data */
 				lowest_prio = rq->rt.highest_prio;
-				if (count) {
-					cpus_clear(*lowest_mask);
-					count = 0;
-				}
+				lowest_cpu = cpu;
+				count = 0;
 			}
 			cpu_set(rq->cpu, *lowest_mask);
 			count++;
+		} else
+			cpu_clear(cpu, *lowest_mask);
+	}
+
+	/*
+	 * Clear out all the set bits that represent
+	 * runqueues that were of higher prio than
+	 * the lowest_prio.
+	 */
+	if (lowest_cpu) {
+		/*
+		 * Perhaps we could add another cpumask op to
+		 * zero out bits. Like cpu_zero_bits(cpumask, nrbits);
+		 * Then that could be optimized to use memset and such.
+		 */
+		for_each_cpu_mask(cpu, *lowest_mask) {
+			if (cpu >= lowest_cpu)
+				break;
+			cpu_clear(cpu, *lowest_mask);
 		}
 	}
 
-
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