Patch "sched/idle/x86: Restore mwait_idle() to fix boot hangs, to improve power savings and to improve performance" has been added to the 4.0-stable tree

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

sched/idle/x86: Restore mwait_idle() to fix boot hangs, to improve power savings and to improve performance

to the 4.0-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-restore-mwait_idle-to-fix-boot-hangs-to-improve-power-savings-and-to-improve-performance.patch
and it can be found in the queue-4.0 subdirectory.

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

From: Len Brown <[email protected]>
Date: Wed, 15 Jan 2014 00:37:34 -0500
Subject: sched/idle/x86: Restore mwait_idle() to fix boot hangs, to improve power savings and to improve performance

From: Len Brown <[email protected]>

commit b253149b843f89cd300cbdbea27ce1f847506f99 upstream.

In Linux-3.9 we removed the mwait_idle() loop:

69fb3676df33 ("x86 idle: remove mwait_idle() and "idle=mwait" cmdline param")

The reasoning was that modern machines should be sufficiently happy during the boot process using the default_idle() HALT loop, until cpuidle loads and either acpi_idle or intel_idle invoke the newer MWAIT-with-hints idle loop.

But two machines reported problems:

1. Certain Core2-era machines support MWAIT-C1 and HALT only. MWAIT-C1 is preferred for optimal power and performance. But if they support just C1, cpuidle never loads and so they use the boot-time default idle loop forever.

2. Some laptops will boot-hang if HALT is used, but will boot successfully if MWAIT is used. This appears to be a hidden assumption in BIOS SMI, that is presumably valid on the proprietary OS where the BIOS was validated.

https://bugzilla.kernel.org/show_bug.cgi?id=60770

So here we effectively revert the patch above, restoring the mwait_idle() loop. However, we don't bother restoring the idle=mwait cmdline parameter, since it appears to add no value.

Maintainer notes:

For 3.9, simply revert 69fb3676df
for 3.10, patch -F3 applies, fuzz needed due to __cpuinit use in context For 3.11, 3.12, 3.13, this patch applies cleanly

---
 arch/x86/include/asm/mwait.h |    8 +++++++
 arch/x86/kernel/process.c    |   47 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)

@@ -30,6 +30,14 @@ static inline void __mwait(unsigned long
 		     :: "a" (eax), "c" (ecx));
 }
 
+static inline void __sti_mwait(unsigned long eax, unsigned long ecx)
+{
+	trace_hardirqs_on();
+	/* "mwait %eax, %ecx;" */
+	asm volatile("sti; .byte 0x0f, 0x01, 0xc9;"
+		     :: "a" (eax), "c" (ecx));
+}
+
 /*
  * This uses new MONITOR/MWAIT instructions on P4 processors with PNI,
  * which can obviate IPI to trigger checking of need_resched.

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

queue-4.0/sched-idle-x86-optimize-unnecessary-mwait_idle-resched-ipis.patch queue-4.0/sched-idle-x86-restore-mwait_idle-to-fix-boot-hangs-to-improve-power-savings-and-to-improve-performance.patch