linux-kernel - Re: [RFC] Extend mwait idle to optimize away IPIs when possible

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Date:	Wed, 8 Feb 2012 14:51:15 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Aaron Durbin <adurbin@...gle.com>,
	Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC] Extend mwait idle to optimize away IPIs when possible

On Mon, Feb 06, 2012 at 12:42:13PM -0800, Venkatesh Pallipadi wrote:
> smp_call_function_single and ttwu_queue_remote sends unconditional IPI
> to target CPU. However, if the target CPU is in mwait based idle, we can
> do IPI-less wakeups using the magical powers of monitor-mwait.
> Doing this has certain advantages:

Actually I'm trying to do the similar thing on MIPS.

The difference is that I want task_is_polling() to do something. The basic
idea is:

> +			if (ipi_pending()) {
> +				clear_ipi_pending();
> +				local_bh_disable();
> +				local_irq_disable();
> +				generic_smp_call_function_single_interrupt();
> +				scheduler_wakeup_self_check();
> +				local_irq_enable();
> +				local_bh_enable();

I let cpu_idle() check if there is anything to do as your above code.

And task_is_polling() handle the others with below patch:
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5255c9d..09f633d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -527,15 +527,16 @@ void resched_task(struct task_struct *p)
 		smp_send_reschedule(cpu);
 }
 
-void resched_cpu(int cpu)
+int resched_cpu(int cpu)
 {
 	struct rq *rq = cpu_rq(cpu);
 	unsigned long flags;
 
 	if (!raw_spin_trylock_irqsave(&rq->lock, flags))
-		return;
+		return 0;
 	resched_task(cpu_curr(cpu));
 	raw_spin_unlock_irqrestore(&rq->lock, flags);
+	return 1;
 }
 
 #ifdef CONFIG_NO_HZ
@@ -1484,7 +1485,8 @@ void scheduler_ipi(void)
 
 static void ttwu_queue_remote(struct task_struct *p, int cpu)
 {
-	if (llist_add(&p->wake_entry, &cpu_rq(cpu)->wake_list))
+	if (llist_add(&p->wake_entry, &cpu_rq(cpu)->wake_list) &&
+						!resched_cpu(cpu))
 		smp_send_reschedule(cpu);
 }
 
Thought?

Thanks,
Yong
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