sched: Move the second half of ttwu() to the remote cpu

Now that we've removed the rq->lock requirement from the first part of
ttwu() and can compute placement without holding any rq->lock, ensure
we execute the second half of ttwu() on the actual cpu we want the
task to run on.

This avoids having to take rq->lock and doing the task enqueue
remotely, saving lots on cacheline transfers.

As measured using: http://oss.oracle.com/~mason/sembench.c

  $ for i in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor ; do echo performance > $i; done
  $ echo 4096 32000 64 128 > /proc/sys/kernel/sem
  $ ./sembench -t 2048 -w 1900 -o 0

  unpatched: run time 30 seconds 647278 worker burns per second
  patched:   run time 30 seconds 816715 worker burns per second

Reviewed-by: Frank Rowand <frank.rowand@am.sony.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20110405152729.515897185@chello.nl

1 files changed, 56 insertions, 0 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 7d8b85fcdf06..9e3ede120e81 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -556,6 +556,10 @@ struct rq {
         unsigned int ttwu_count;
         unsigned int ttwu_local;
 #endif
+
+#ifdef CONFIG_SMP
+        struct task_struct *wake_list;
+#endif
 };
 
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);
@@ -2516,10 +2520,61 @@ static int ttwu_remote(struct task_struct *p, int wake_flags)
         return ret;
 }
 
+#ifdef CONFIG_SMP
+static void sched_ttwu_pending(void)
+{
+        struct rq *rq = this_rq();
+        struct task_struct *list = xchg(&rq->wake_list, NULL);
+
+        if (!list)
+                return;
+
+        raw_spin_lock(&rq->lock);
+
+        while (list) {
+                struct task_struct *p = list;
+                list = list->wake_entry;
+                ttwu_do_activate(rq, p, 0);
+        }
+
+        raw_spin_unlock(&rq->lock);
+}
+
+void scheduler_ipi(void)
+{
+        sched_ttwu_pending();
+}
+
+static void ttwu_queue_remote(struct task_struct *p, int cpu)
+{
+        struct rq *rq = cpu_rq(cpu);
+        struct task_struct *next = rq->wake_list;
+
+        for (;;) {
+                struct task_struct *old = next;
+
+                p->wake_entry = next;
+                next = cmpxchg(&rq->wake_list, old, p);
+                if (next == old)
+                        break;
+        }
+
+        if (!next)
+                smp_send_reschedule(cpu);
+}
+#endif
+
 static void ttwu_queue(struct task_struct *p, int cpu)
 {
         struct rq *rq = cpu_rq(cpu);
 
+#if defined(CONFIG_SMP) && defined(CONFIG_SCHED_TTWU_QUEUE)
+        if (sched_feat(TTWU_QUEUE) && cpu != smp_processor_id()) {
+                ttwu_queue_remote(p, cpu);
+                return;
+        }
+#endif
+
         raw_spin_lock(&rq->lock);
         ttwu_do_activate(rq, p, 0);
         raw_spin_unlock(&rq->lock);
@@ -6331,6 +6386,7 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
 
 #ifdef CONFIG_HOTPLUG_CPU
         case CPU_DYING:
+                sched_ttwu_pending();
                 /* Update our root-domain */
                 raw_spin_lock_irqsave(&rq->lock, flags);
                 if (rq->rd) {