linux-kernel - [PATCH 1/1] scheduler: minor improvement to pick_next_highest_task

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Date:	Mon, 19 Mar 2012 22:26:19 +0000
From:	"Michael J. Wang" <mjwang@...adcom.com>
To:	"mingo@...e.hu" <mingo@...e.hu>,
	"peterz@...radead.org" <peterz@...radead.org>
cc:	"Michael J. Wang" <mjwang@...adcom.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"yong.zhang0@...il.com" <yong.zhang0@...il.com>
Subject: [PATCH 1/1] scheduler: minor improvement to
 pick_next_highest_task_rt in linux-3.3

From: Michael J Wang <mjwang@...adcom.com>

Avoid extra work by continuing on to the next rt_rq if the highest prio task in current rt_rq is the same priority as our candidate task.

Signed-off-by: Michael J Wang <mjwang@...adcom.com>

---

More detailed explanation:  if next is not NULL, then we have found a candidate task, and its priority is next->prio.  Now we are looking for an even higher priority task in the other rt_rq's.  idx is the highest priority in the current candidate rt_rq.  In the current 3.3 code, if idx is equal to next->prio, we would start scanning the tasks in that rt_rq and replace the current candidate task with a task from that rt_rq.  But the new task would only have a priority that is equal to our previous candidate task, so we have not advanced our goal of finding a higher prio task.  So we should avoid the extra work by continuing on to the next rt_rq if idx is equal to next->prio.

--- linux-3.3/kernel/sched/rt.c.orig	2012-03-18 16:15:34.000000000 -0700
+++ linux-3.3/kernel/sched/rt.c	2012-03-19 14:52:54.585391702 -0700
@@ -1403,7 +1403,7 @@ static struct task_struct *pick_next_hig
 next_idx:
 		if (idx >= MAX_RT_PRIO)
 			continue;
-		if (next && next->prio < idx)
+		if (next && next->prio <= idx)
 			continue;
 		list_for_each_entry(rt_se, array->queue + idx, run_list) {
 			struct task_struct *p;

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