linux-kernel - Re: [PATCH] sched: fair scheduler should not resched rt tasks

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Date:	Sat, 11 Oct 2008 13:02:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <clark.williams@...il.com>
Subject: Re: [PATCH] sched: fair scheduler should not resched rt tasks

On Sat, 2008-10-11 at 03:01 -0400, Steven Rostedt wrote:
> Using ftrace, I noticed latencies in real-time tasks where they were
> needlessly calling schedule due to sched_fair sending out time slices.
> 
> This patch prevents a call to resched_task by the sched fair class if
> the task it wants to reschedule is an rt task.

Right, thats not a good thing, however this patch looks wrong, we should
never call hrtick_start_fair() on a rt task to begin with.

The way I can see that happening is through enqueue/dequeue_task_fair()
where we want to re-programm the hrtick because nr_running changes (and
thus the current tasks desired runtime).

Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
---
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 5d39d92..6bcceec 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -73,6 +73,8 @@ unsigned int sysctl_sched_wakeup_granularity = 5000000UL;
 
 const_debug unsigned int sysctl_sched_migration_cost = 500000UL;
 
+static const struct sched_class fair_sched_class;
+
 /**************************************************************
  * CFS operations on generic schedulable entities:
  */
@@ -849,11 +851,31 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
 		hrtick_start(rq, delta);
 	}
 }
+
+/*
+ * called from enqueue/dequeue and updates the hrtick when the
+ * current task is from our class and nr_running is low enough
+ * to matter.
+ */
+static void hrtick_update(struct rq *rq)
+{
+	struct task_struct *curr = rq->curr;
+
+	if (curr->sched_class != &fair_sched_class)
+		return;
+
+	if (cfs_rq_of(&curr->se)->nr_running < sched_nr_latency)
+		hrtick_start_fair(rq, curr);
+}
 #else /* !CONFIG_SCHED_HRTICK */
 static inline void
 hrtick_start_fair(struct rq *rq, struct task_struct *p)
 {
 }
+
+static inline void hrtick_update(struct rq *rq)
+{
+}
 #endif
 
 /*
@@ -874,7 +896,7 @@ static void enqueue_task_fair(struct rq *rq, struct task_struct *p, int wakeup)
 		wakeup = 1;
 	}
 
-	hrtick_start_fair(rq, rq->curr);
+	hrtick_update(rq);
 }
 
 /*
@@ -896,7 +918,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int sleep)
 		sleep = 1;
 	}
 
-	hrtick_start_fair(rq, rq->curr);
+	hrtick_update(rq);
 }
 
 /*
@@ -1002,8 +1024,6 @@ static inline int wake_idle(int cpu, struct task_struct *p)
 
 #ifdef CONFIG_SMP
 
-static const struct sched_class fair_sched_class;
-
 #ifdef CONFIG_FAIR_GROUP_SCHED
 /*
  * effective_load() calculates the load change as seen from the root_task_group


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