[PATCH] sched: filter affine wakeups

)

From: Nick Piggin <nickpiggin@yahoo.com.au>

Track the last waker CPU, and only consider wakeup-balancing if there's a
match between current waker CPU and the previous waker CPU.  This ensures
that there is some correlation between two subsequent wakeup events before
we move the task.  Should help random-wakeup workloads on large SMP
systems, by reducing the migration attempts by a factor of nr_cpus.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 13 insertions, 2 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 5d6b9228bba9..b5ef92a043a6 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -696,9 +696,12 @@ struct task_struct {
 
         int lock_depth;         /* BKL lock depth */
 
-#if defined(CONFIG_SMP) && defined(__ARCH_WANT_UNLOCKED_CTXSW)
+#if defined(CONFIG_SMP)
+        int last_waker_cpu;     /* CPU that last woke this task up */
+#if defined(__ARCH_WANT_UNLOCKED_CTXSW)
         int oncpu;
 #endif
+#endif
         int prio, static_prio;
         struct list_head run_list;
         prio_array_t *array;
diff --git a/kernel/sched.c b/kernel/sched.c
index 98461de1ab65..c9dec2aa1976 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1290,6 +1290,9 @@ static int try_to_wake_up(task_t *p, unsigned int state, int sync)
                 }
         }
 
+        if (p->last_waker_cpu != this_cpu)
+                goto out_set_cpu;
+
         if (unlikely(!cpu_isset(this_cpu, p->cpus_allowed)))
                 goto out_set_cpu;
 
@@ -1360,6 +1363,8 @@ out_set_cpu:
                 cpu = task_cpu(p);
         }
 
+        p->last_waker_cpu = this_cpu;
+
 out_activate:
 #endif /* CONFIG_SMP */
         if (old_state == TASK_UNINTERRUPTIBLE) {
@@ -1441,9 +1446,12 @@ void fastcall sched_fork(task_t *p, int clone_flags)
 #ifdef CONFIG_SCHEDSTATS
         memset(&p->sched_info, 0, sizeof(p->sched_info));
 #endif
-#if defined(CONFIG_SMP) && defined(__ARCH_WANT_UNLOCKED_CTXSW)
+#if defined(CONFIG_SMP)
+        p->last_waker_cpu = cpu;
+#if defined(__ARCH_WANT_UNLOCKED_CTXSW)
         p->oncpu = 0;
 #endif
+#endif
 #ifdef CONFIG_PREEMPT
         /* Want to start with kernel preemption disabled. */
         task_thread_info(p)->preempt_count = 1;