linux-kernel - Re: [PATCH REGRESSION FIX] x86 idle: restore mwait

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Date:	Sat, 18 Jan 2014 17:14:44 +0100
From:	Mike Galbraith <bitbucket@...ine.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Len Brown <lenb@...nel.org>, x86@...nel.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Len Brown <len.brown@...el.com>,
	Ian Malone <ibmalone@...il.com>,
	Josh Boyer <jwboyer@...hat.com>, stable@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH REGRESSION FIX] x86 idle: restore mwait_idle()

On Sat, 2014-01-18 at 10:33 +0100, Mike Galbraith wrote: 
> On Fri, 2014-01-17 at 05:20 +0100, Mike Galbraith wrote:
> 
> > taskset 0xc pipe-test 1
> > 
> > 3.8.13     3.397977 usecs/loop -- avg 3.400336 588.2 KHz
> > master+    4.798547 usecs/loop -- avg 4.791692 417.4 KHz
> 
> Bah, those are apple/grape, these are apple/apple.

Or, to make it more correct for 3.10..13, add the clflush barriers as
well for afflicted CPUs.

idle: kill unnecessary mwait_idle() resched IPIs

Set/clear polling instead.

Q6600, pipe-test scheduling cross core:
3.8.13                   487.2 KHz    1.000
3.13.0-master            415.5 KHz     .852
3.13.0-master+           415.2 KHz     .852     + restore mwait_idle
3.13.0-master++          488.5 KHz    1.002     + restore mwait_idle + IPI fix

Signed-off-by: Mike Galbraith <bitbucket@...ine.de>
Cc: <stable@...r.kernel.org> # 3.10+
---
 arch/x86/kernel/process.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -427,18 +427,21 @@ static int prefer_mwait_c1_over_halt(con
 
 static void mwait_idle(void)
 {
-	if (!need_resched()) {
-		if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR))
+	if (!current_set_polling_and_test()) {
+		if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR)) {
+			mb();
 			clflush((void *)&current_thread_info()->flags);
+			mb();
+		}
 
 		__monitor((void *)&current_thread_info()->flags, 0, 0);
-		smp_mb();
 		if (!need_resched())
 			__sti_mwait(0, 0);
 		else
 			local_irq_enable();
 	} else
 		local_irq_enable();
+	__current_clr_polling();
 }
 
 void select_idle_routine(const struct cpuinfo_x86 *c)


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