[PATCH 3.10 06/65] sched/idle/x86: Restore mwait_idle() to fix boot hangs, to improve power savings and to improve performance

[Greg Kroah-Hartman]

3.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Len Brown <len.br...@intel.com>

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

Tested-by: Mike Galbraith <bitbuc...@online.de>
Signed-off-by: Len Brown <len.br...@intel.com>
Acked-by: Mike Galbraith <bitbuc...@online.de>
Cc: <sta...@vger.kernel.org> # 3.9+
Cc: Borislav Petkov <b...@alien8.de>
Cc: H. Peter Anvin <h...@zytor.com>
Cc: Ian Malone <ibmal...@gmail.com>
Cc: Josh Boyer <jwbo...@redhat.com>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Mike Galbraith <efa...@gmx.de>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Link: 
http://lkml.kernel.org/r/345254a551eb5a6a866e048d7ab570fd2193aca4.1389763084.git.len.br...@intel.com
[ Ported to recent kernels. ]
[ Mike: 3.10 backport ]
Signed-off-by: Ingo Molnar <mi...@kernel.org>
Signed-off-by: Mike Galbraith <efa...@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 arch/x86/kernel/process.c |   49 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -398,6 +398,52 @@ 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 (!current_set_polling_and_test()) {
+               if (static_cpu_has(X86_FEATURE_CLFLUSH_MONITOR)) {
+                       mb();
+                       clflush((void *)&current_thread_info()->flags);
+                       mb();
+               }
+
+               __monitor((void *)&current_thread_info()->flags, 0, 0);
+               if (!need_resched())
+                       __sti_mwait(0, 0);
+               else
+                       local_irq_enable();
+       } else
+               local_irq_enable();
+       __current_clr_polling();
+}
+
 void __cpuinit select_idle_routine(const struct cpuinfo_x86 *c)
 {
 #ifdef CONFIG_SMP
@@ -411,6 +457,9 @@ void __cpuinit select_idle_routine(const
                /* 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;
 }


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