[PATCH 20/22] sched: Move the second half of ttwu() to the remote cpu

Peter Zijlstra
2011-03-02T17:46:26Z

[-- Attachment #1: sched-ttwu-queue-remote.patch --]
[-- Type: text/plain, Size: 3766 bytes --]

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

$ echo 4096 32000 64 128 > /proc/sys/kernel/sem
$ ./sembench -t 2048 -w 1900 -o 0

unpatched: run time 30 seconds 537953 worker burns per second
patched: run time 30 seconds 847526 worker burns per second

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
---
include/linux/sched.h | 4 +--
kernel/sched.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++
kernel/sched_features.h | 2 +
3 files changed, 60 insertions(+), 2 deletions(-)

Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -1202,6 +1201,7 @@ struct task_struct {
int lock_depth; /* BKL lock depth */

#ifdef CONFIG_SMP
+ struct task_struct *wake_entry;
int on_cpu;
#endif
int on_rq;
@@ -2185,7 +2185,7 @@ extern void set_task_comm(struct task_st
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 unsigned long wait_task_inactive(struct task_struct *p,
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -557,6 +557,10 @@ 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);
@@ -2511,10 +2515,61 @@ static int ttwu_remote(struct task_struc
return ret;
}

+#ifdef CONFIG_SMP
+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);

+#ifdef CONFIG_SMP
+ if (!sched_feat(TTWU_FORCE_REMOTE) && 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);
@@ -6287,6 +6342,7 @@ migration_call(struct notifier_block *nf

#ifdef CONFIG_HOTPLUG_CPU
case CPU_DYING:
+ sched_ttwu_pending();
/* Update our root-domain */
raw_spin_lock_irqsave(&rq->lock, flags);
if (rq->rd) {
Index: linux-2.6/kernel/sched_features.h
===================================================================
--- linux-2.6.orig/kernel/sched_features.h
+++ linux-2.6/kernel/sched_features.h
@@ -64,3 +64,5 @@ SCHED_FEAT(OWNER_SPIN, 1)
* Decrement CPU power based on irq activity
*/
SCHED_FEAT(NONIRQ_POWER, 1)
+
+SCHED_FEAT(TTWU_FORCE_REMOTE, 0)