On Sat, 2007-09-22 at 12:01 +0200, Mike Galbraith wrote:
quoted text
> On Fri, 2007-09-21 at 20:27 -0700, Tong Li wrote:
> > Mike,
> > 
> > Could you try this patch to see if it solves the latency problem?
> 
> No, but it helps some when running two un-pinned busy loops, one at nice
> 0, and the other at nice 19.  Yesterday I hit latencies of up to 1.2
> _seconds_ doing this, and logging sched_debug and /proc/`pidof
> Xorg`/sched from SCHED_RR shells.

Looking at a log (snippet attached) from this morning with the last hunk
of your patch still intact, it looks like min_vruntime is being modified
after a task is queued.  If you look at the snippet, you'll see the nice
19 bash busy loop on CPU1 with a vruntime of 3010385.345325, and one
second later on CPU1 with it's vruntime at 2814952.425082, but
min_vruntime is 3061874.838356.

I took a hammer to it, and my latencies running this test went away.

diff -uprNX /root/dontdiff git/linux-2.6.sched-devel/kernel/sched_fair.c linux-2.6.23-rc7.d/kernel/sched_fair.c
--- git/linux-2.6.sched-devel/kernel/sched_fair.c	2007-09-22 13:37:32.000000000 +0200
+++ linux-2.6.23-rc7.d/kernel/sched_fair.c	2007-09-23 08:29:38.000000000 +0200
@@ -290,14 +290,19 @@ __update_curr(struct cfs_rq *cfs_rq, str
 static void sync_vruntime(struct cfs_rq *cfs_rq)
 {
 	struct rq *rq;
-	int cpu;
+	int cpu, wrote = 0;
 
 	for_each_online_cpu(cpu) {
 		rq = &per_cpu(runqueues, cpu);
+		if (spin_is_locked(&rq->lock))
+			continue;
+		smp_wmb();
 		cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime,
 				rq->cfs.min_vruntime);
+		wrote++;
 	}
-	schedstat_inc(cfs_rq, nr_sync_min_vruntime);
+	if (wrote)
+		schedstat_inc(cfs_rq, nr_sync_min_vruntime);
 }
 
 static void update_curr(struct cfs_rq *cfs_rq)
@@ -306,8 +311,10 @@ static void update_curr(struct cfs_rq *c
 	u64 now = rq_of(cfs_rq)->clock;
 	unsigned long delta_exec;
 
-	if (unlikely(!curr))
+	if (unlikely(!cfs_rq->nr_running))
 		return sync_vruntime(cfs_rq);
+	if (unlikely(!curr))
+		return;
 
 	/*
 	 * Get the amount of time the current task was running