Re: [PATCH RFC] smt nice introduces significant lock contention

[Nick Piggin <nickpiggin@yahoo.com.au>]

Con Kolivas wrote:
> On Friday 02 June 2006 17:53, Nick Piggin wrote:
>
> > This is a small micro-optimisation / cleanup we can do after
> > smtnice gets converted to use trylocks. Might result in a little
> > less cacheline footprint in some cases.
>
>
> It's only dependent_sleeper that is being converted in these patches. The 
> wake_sleeping_dependent component still locks all runqueues and needs to 

Oh I missed that.

> succeed in order to ensure a task doesn't keep sleeping indefinitely. That 

Let's make it use trylocks as well. wake_priority_sleeper should ensure
things don't sleep forever I think? We should be optimising for the most
common case, and in many workloads, the runqueue does go idle frequently.

> one doesn't get called from schedule() so is far less expensive. This means I 
> don't think we can change that cpu based locking order which I believe was 
> introduce to prevent a deadlock (?DaveJ disovered it iirc).

AntonB, I think.

--
SUSE Labs, Novell Inc.

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c	2006-06-02 18:23:18.000000000 +1000
+++ linux-2.6/kernel/sched.c	2006-06-02 18:26:40.000000000 +1000
@@ -2686,6 +2686,9 @@ static inline void wakeup_busy_runqueue(
 		resched_task(rq->idle);
 }
 
+/*
+ * Called with interrupts disabled and this_rq's runqueue locked.
+ */
 static void wake_sleeping_dependent(int this_cpu, runqueue_t *this_rq)
 {
 	struct sched_domain *tmp, *sd = NULL;
@@ -2699,22 +2702,13 @@ static void wake_sleeping_dependent(int 
 	if (!sd)
 		return;
 
-	/*
-	 * Unlock the current runqueue because we have to lock in
-	 * CPU order to avoid deadlocks. Caller knows that we might
-	 * unlock. We keep IRQs disabled.
-	 */
-	spin_unlock(&this_rq->lock);
-
 	sibling_map = sd->span;
-
-	for_each_cpu_mask(i, sibling_map)
-		spin_lock(&cpu_rq(i)->lock);
-	/*
-	 * We clear this CPU from the mask. This both simplifies the
-	 * inner loop and keps this_rq locked when we exit:
-	 */
 	cpu_clear(this_cpu, sibling_map);
+	for_each_cpu_mask(i, sibling_map) {
+		if (unlikely(!spin_trylock(&cpu_rq(i)->lock)))
+			cpu_clear(i, sibling_map);
+	}
+
 
 	for_each_cpu_mask(i, sibling_map) {
 		runqueue_t *smt_rq = cpu_rq(i);
@@ -2724,10 +2718,6 @@ static void wake_sleeping_dependent(int 
 
 	for_each_cpu_mask(i, sibling_map)
 		spin_unlock(&cpu_rq(i)->lock);
-	/*
-	 * We exit with this_cpu's rq still held and IRQs
-	 * still disabled:
-	 */
 }
 
 /*
@@ -2961,13 +2951,6 @@ need_resched_nonpreemptible:
 			next = rq->idle;
 			rq->expired_timestamp = 0;
 			wake_sleeping_dependent(cpu, rq);
-			/*
-			 * wake_sleeping_dependent() might have released
-			 * the runqueue, so break out if we got new
-			 * tasks meanwhile:
-			 */
-			if (!rq->nr_running)
-				goto switch_tasks;
 		}
 	}