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;
}
}
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