Commit 317f3941 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

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: default avatarFrank 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: default avatarIngo Molnar <mingo@elte.hu>
Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20110405152729.515897185@chello.nl
parent c05fbafb

include/linux/sched.h

+2 −1
Original line number Diff line number Diff line
@@ -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) { }

init/Kconfig

+5 −0
Original line number Diff line number Diff line
@@ -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

kernel/sched.c

+56 −0
Original line number Diff line number Diff line
@@ -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) {

kernel/sched_features.h

+6 −0
Original line number Diff line number Diff line
@@ -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)

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