Patch "sched/idle/x86: Optimize unnecessary mwait_idle() resched IPIs" has been added to the 3.19-stable tree

This is a note to let you know that I've just added the patch titled

sched/idle/x86: Optimize unnecessary mwait_idle() resched IPIs

to the 3.19-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sched-idle-x86-optimize-unnecessary-mwait_idle-resched-ipis.patch
and it can be found in the queue-3.19 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Mike Galbraith <[email protected]>
Date: Sat, 18 Jan 2014 17:14:44 +0100
Subject: sched/idle/x86: Optimize unnecessary mwait_idle() resched IPIs

From: Mike Galbraith <[email protected]>

commit f8e617f4582995f7c25ef25b4167213120ad122b upstream.

To fully take advantage of MWAIT, apparently the CLFLUSH instruction needs another quirk on certain CPUs: proper barriers around it on certain machines.

On a Q6600 SMP system, pipe-test scheduling performance, cross core, improves significantly:

  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

Since X86_BUG_CLFLUSH_MONITOR is already a quirk, don't create a separate quirk for the extra smp_mb()s.

---
 arch/x86/kernel/process.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

@@ -428,18 +428,22 @@ static int prefer_mwait_c1_over_halt(con
 
 static void mwait_idle(void)
 {
-	if (!need_resched()) {
-		if (this_cpu_has(X86_BUG_CLFLUSH_MONITOR))
+	if (!current_set_polling_and_test()) {
+		if (this_cpu_has(X86_BUG_CLFLUSH_MONITOR)) {
+			smp_mb(); /* quirk */
 			clflush((void *)&current_thread_info()->flags);
+			smp_mb(); /* quirk */
+		}
 
 		__monitor((void *)&current_thread_info()->flags, 0, 0);
-		smp_mb();
 		if (!need_resched())
 			__sti_mwait(0, 0);
 		else
 			local_irq_enable();
-	} else
+	} else {
 		local_irq_enable();
+	}
+	__current_clr_polling();
 }
 
 void select_idle_routine(const struct cpuinfo_x86 *c)

Patches currently in stable-queue which might be from [email protected] are

queue-3.19/sched-idle-x86-optimize-unnecessary-mwait_idle-resched-ipis.patch queue-3.19/sched-idle-x86-restore-mwait_idle-to-fix-boot-hangs-to-improve-power-savings-and-to-improve-performance.patch