[PATCH REGRESSION FIX] x86 idle: restore mwait_idle()

[Len Brown]

From: Len Brown <len.brown@xxxxxxxxx>

In Linux-3.9 we removed the mwait_idle() loop:
'x86 idle: remove mwait_idle() and "idle=mwait" cmdline param'
(69fb3676df3329a7142803bb3502fa59dc0db2e3)

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

Cc: Mike Galbraith <bitbucket@xxxxxxxxx>
Cc: Ian Malone <ibmalone@xxxxxxxxx>
Cc: Josh Boyer <jwboyer@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 3.9, 3.10, 3.11, 3.12, 3.13
Signed-off-by: Len Brown <len.brown@xxxxxxxxx>
---
 arch/x86/kernel/process.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 3fb8d95..db471a8 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -398,6 +398,49 @@ static void amd_e400_idle(void)
 		default_idle();
 }
 
+/*
+ * Intel Core2 and older machines prefer MWAIT over HALT for C1.
+ * We can't rely on cpuidle installing MWAIT, because it will not load
+ * on systems that support only C1 -- so the boot default must be MWAIT.
+ *  
+ * Some AMD machines are the opposite, they depend on using HALT.
+ *
+ * So for default C1, which is used during boot until cpuidle loads,
+ * use MWAIT-C1 on Intel HW that has it, else use HALT.
+ */
+static int prefer_mwait_c1_over_halt(const struct cpuinfo_x86 *c)
+{
+	if (c->x86_vendor != X86_VENDOR_INTEL)
+		return 0;
+
+	if (!cpu_has(c, X86_FEATURE_MWAIT))
+		return 0;
+
+	return 1;
+}
+
+/*
+ * MONITOR/MWAIT with no hints, used for default default C1 state.
+ * This invokes MWAIT with interrutps enabled and no flags,
+ * which is backwards compatible with the original MWAIT implementation.
+ */
+
+static void mwait_idle(void)
+{
+	if (!need_resched()) {
+		if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR))
+			clflush((void *)&current_thread_info()->flags);
+
+		__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();
+}
+
 void select_idle_routine(const struct cpuinfo_x86 *c)
 {
 #ifdef CONFIG_SMP
@@ -411,6 +454,9 @@ void select_idle_routine(const struct cpuinfo_x86 *c)
 		/* E400: APIC timer interrupt does not wake up CPU from C1e */
 		pr_info("using AMD E400 aware idle routine\n");
 		x86_idle = amd_e400_idle;
+	} else if (prefer_mwait_c1_over_halt(c)) {
+		pr_info("using mwait in idle threads\n");
+		x86_idle = mwait_idle;
 	} else
 		x86_idle = default_idle;
 }
-- 
1.8.5.2.309.ga25014b

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