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Date: Sat, 12 Apr 2014 10:35:02 +0200
From: Mike Galbraith <umgwanakikbuti@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...nel.org, tglx@...utronix.de, luto@...capital.net,
nicolas.pitre@...aro.org, daniel.lezcano@...aro.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 0/8] sched,idle: need resched polling rework
On Fri, 2014-04-11 at 15:42 +0200, Peter Zijlstra wrote:
> A while ago both Mike and Andy complained that we still get pointless wakeup
> IPIs, we had a few patches back and forth but eventually more or less agreed
> and then nothing... :-)
>
> So here's a number of patches that implement something near what we left off
> with.
>
> Its only been compile/boot tested on x86_64, I've no actually looked at the IPI
> numbers yet.
'course this didn't do much for my Q6600 box, or core2 lappy when booted
max_cstate=1, but series didn't seem to break anything, both boxen still
work fine with the below on top.
Subject: [PATCH REGRESSION FIX] x86 idle: restore mwait_idle()
From: Len Brown <lenb@...nel.org>
Date: Wed, 15 Jan 2014 00:37:34 -0500
From: Len Brown <len.brown@...el.com>
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
Mike: add clflush barriers and resched IPI avoidance.
Cc: Mike Galbraith <bitbucket@...ine.de>
Cc: Ian Malone <ibmalone@...il.com>
Cc: Josh Boyer <jwboyer@...hat.com>
Cc: <stable@...r.kernel.org> # 3.9, 3.10, 3.11, 3.12, 3.13
Signed-off-by: Len Brown <len.brown@...el.com>
Signed-off-by: Mike Galbraith <bitbucket@...ine.de>
---
arch/x86/include/asm/mwait.h | 8 ++++++
arch/x86/kernel/process.c | 50 +++++++++++++++++++++++++++++++++++++++++++
2 files changed, 58 insertions(+)
--- a/arch/x86/include/asm/mwait.h
+++ b/arch/x86/include/asm/mwait.h
@@ -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.
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -28,6 +28,7 @@
#include <asm/fpu-internal.h>
#include <asm/debugreg.h>
#include <asm/nmi.h>
+#include <asm/mwait.h>
/*
* per-CPU TSS segments. Threads are completely 'soft' on Linux,
@@ -395,6 +396,52 @@ 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 (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR)) {
+ mb();
+ clflush((void *)¤t_thread_info()->flags);
+ mb();
+ }
+
+ __monitor((void *)¤t_thread_info()->flags, 0, 0);
+ 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)
{
#ifdef CONFIG_SMP
@@ -408,6 +455,9 @@ void select_idle_routine(const struct cp
/* 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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