Re: 2.6.17-rc2+ regression -- audio skipping

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Lee Revell wrote:

On Sun, 2006-05-21 at 22:24 +0200, Rene Herman wrote:

2.6.17-rc2 (and 3 and 4) make my audio skip. Audio player is ogg123 running in an xterm. Browsing heavy sites (say, eBay) with Firefox 1.5.0.3 gets me audio underruns quickly. This does not happen on 2.6.17-rc1 and earlier (I just tested extensively; quite impossible to generate underruns on -rc1, quickly on -rc2 and later).

It's not ALSA; reverted */sound/* from the rc1-rc2 interdiff. It's also not cfq-iosched.c. Any likely culprits in there? (I'm not a GIT user).


I would suspect the scheduler interactivity patches.  Please confirm
this by running ogg123 at nice -20 - do the underruns persist?

They do persist. Thanks for the hint though -- "sched: fix interactive task starvation" is the culprit:

http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=5ce74abe788a26698876e66b9c9ce7e7acc25413

Added author and acked-by's to the CC. Mike, this patch is no good for me. Audio underruns galore, with only ogg123 and firefox (browsing the GIT tree online is also a nice trigger by the way).

If I back it out, everything is fine for me again. Back-out attached as a patch against -rc4. This also backs out your follow-up "don't awaken RT tasks on expired array" as it was dependant:

http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=8a5bc075b8d8cf7a87b3f08fad2fba0f5d13295e

While looking at the patch I noticed there was +1 difference in the "limit" value between the macro and the static inline version of expired_starving() so I experimented with adding that back but that wasn't it unfortunately.

I can test patches (preferably versus -rc4) although possibly not quickly.

Rene.

Index: local/kernel/sched.c
===================================================================
--- local.orig/kernel/sched.c	2006-05-08 20:47:06.000000000 +0200
+++ local/kernel/sched.c	2006-05-22 01:03:12.000000000 +0200
@@ -665,55 +665,13 @@ static int effective_prio(task_t *p)
 }
 
 /*
- * We place interactive tasks back into the active array, if possible.
- *
- * To guarantee that this does not starve expired tasks we ignore the
- * interactivity of a task if the first expired task had to wait more
- * than a 'reasonable' amount of time. This deadline timeout is
- * load-dependent, as the frequency of array switched decreases with
- * increasing number of running tasks. We also ignore the interactivity
- * if a better static_prio task has expired, and switch periodically
- * regardless, to ensure that highly interactive tasks do not starve
- * the less fortunate for unreasonably long periods.
- */
-static inline int expired_starving(runqueue_t *rq)
-{
-	int limit;
-
-	/*
-	 * Arrays were recently switched, all is well
-	 */
-	if (!rq->expired_timestamp)
-		return 0;
-
-	limit = STARVATION_LIMIT * rq->nr_running;
-
-	/*
-	 * It's time to switch arrays
-	 */
-	if (jiffies - rq->expired_timestamp >= limit)
-		return 1;
-
-	/*
-	 * There's a better selection in the expired array
-	 */
-	if (rq->curr->static_prio > rq->best_expired_prio)
-		return 1;
-
-	/*
-	 * All is well
-	 */
-	return 0;
-}
-
-/*
  * __activate_task - move a task to the runqueue.
  */
 static void __activate_task(task_t *p, runqueue_t *rq)
 {
 	prio_array_t *target = rq->active;
 
-	if (unlikely(batch_task(p) || (expired_starving(rq) && !rt_task(p))))
+	if (batch_task(p))
 		target = rq->expired;
 	enqueue_task(p, target);
 	rq->nr_running++;
@@ -2532,6 +2490,22 @@ unsigned long long current_sched_time(co
 }
 
 /*
+ * We place interactive tasks back into the active array, if possible.
+ *
+ * To guarantee that this does not starve expired tasks we ignore the
+ * interactivity of a task if the first expired task had to wait more
+ * than a 'reasonable' amount of time. This deadline timeout is
+ * load-dependent, as the frequency of array switched decreases with
+ * increasing number of running tasks. We also ignore the interactivity
+ * if a better static_prio task has expired:
+ */
+#define EXPIRED_STARVING(rq) \
+	((STARVATION_LIMIT && ((rq)->expired_timestamp && \
+		(jiffies - (rq)->expired_timestamp >= \
+			STARVATION_LIMIT * ((rq)->nr_running) + 1))) || \
+			((rq)->curr->static_prio > (rq)->best_expired_prio))
+
+/*
  * Account user cpu time to a process.
  * @p: the process that the cpu time gets accounted to
  * @hardirq_offset: the offset to subtract from hardirq_count()
@@ -2666,7 +2640,7 @@ void scheduler_tick(void)
 
 		if (!rq->expired_timestamp)
 			rq->expired_timestamp = jiffies;
-		if (!TASK_INTERACTIVE(p) || expired_starving(rq)) {
+		if (!TASK_INTERACTIVE(p) || EXPIRED_STARVING(rq)) {
 			enqueue_task(p, rq->expired);
 			if (p->static_prio < rq->best_expired_prio)
 				rq->best_expired_prio = p->static_prio;

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