linux-kernel - [PATCH]sched/rt: Do not account zero delta_exec in update_curr

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Date:	Wed, 30 Jan 2013 16:50:36 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: [PATCH]sched/rt: Do not account zero delta_exec in update_curr_rt()

There are several places of consecutive calls of dequeue_task_rt()
and put_prev_task_rt() in the scheduler. For example, function
rt_mutex_setprio() does it.

The both calls lead to update_curr_rt(), the second of it receives
zeroed delta_exec. The only effective action in this case is call of
sched_rt_avg_update(), which can change rq->age_stamp and rq->rt_avg.
But it is possible in case of ""floating"" rq->clock. This fact is
not reasonable to be accounted. Another actions do nothing.

Signed-off-by: Kirill V Tkhai <tkhai@...dex.ru>
CC: Steven Rostedt <rostedt@...dmis.org>
CC: Ingo Molnar <mingo@...nel.org>
CC: Peter Zijlstra <peterz@...radead.org>
CC: linux-rt-users <linux-rt-users@...r.kernel.org>
---
 kernel/sched/rt.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 4e8f0f4..c6d7993 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -925,8 +925,8 @@ static void update_curr_rt(struct rq *rq)
 		return;
 
 	delta_exec = rq->clock_task - curr->se.exec_start;
-	if (unlikely((s64)delta_exec < 0))
-		delta_exec = 0;
+	if (unlikely((s64)delta_exec <= 0))
+		return;
 
 	schedstat_set(curr->se.statistics.exec_max,
 		      max(curr->se.statistics.exec_max, delta_exec));

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