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

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 25c5031..e09dafa 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1203,6 +1203,7 @@ struct task_struct {
        int lock_depth;         /* BKL lock depth */
 
 #ifdef CONFIG_SMP
+       struct task_struct *wake_entry;
        int on_cpu;
 #endif
        int on_rq;
@@ -2192,7 +2193,7 @@ extern void set_task_comm(struct task_struct *tsk, char *from);
 extern char *get_task_comm(char *to, struct task_struct *tsk);
 
 #ifdef CONFIG_SMP
-static inline void scheduler_ipi(void) { }
+void scheduler_ipi(void);
 extern unsigned long wait_task_inactive(struct task_struct *, long match_state);
 #else
 static inline void scheduler_ipi(void) { }

diff --git a/init/Kconfig b/init/Kconfig
index 56240e7..32745bf 100644 (file)
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -827,6 +827,11 @@ config SCHED_AUTOGROUP
          desktop applications.  Task group autogeneration is currently based
          upon task session.
 
+config SCHED_TTWU_QUEUE
+       bool
+       depends on !SPARC32
+       default y
+
 config MM_OWNER
        bool
 

diff --git a/kernel/sched.c b/kernel/sched.c
index 7d8b85f..9e3ede1 100644 (file)
--- 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) {

diff --git a/kernel/sched_features.h b/kernel/sched_features.h
index 68e69ac..be40f73 100644 (file)
--- a/kernel/sched_features.h
+++ b/kernel/sched_features.h
@@ -64,3 +64,9 @@ SCHED_FEAT(OWNER_SPIN, 1)
  * Decrement CPU power based on irq activity
  */
 SCHED_FEAT(NONIRQ_POWER, 1)
+
+/*
+ * Queue remote wakeups on the target CPU and process them
+ * using the scheduler IPI. Reduces rq->lock contention/bounces.
+ */
+SCHED_FEAT(TTWU_QUEUE, 1)