linux-kernel - [PATCH -v5 5/5] x86,smp: limit spinlock delay on virtual machines

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Date:	Wed, 6 Feb 2013 15:07:48 -0500
From:	Rik van Riel <riel@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	aquini@...hat.com, eric.dumazet@...il.com, lwoodman@...hat.com,
	knoel@...hat.com, chegu_vinod@...com,
	raghavendra.kt@...ux.vnet.ibm.com, mingo@...hat.com
Subject: [PATCH -v5 5/5] x86,smp: limit spinlock delay on virtual machines

Modern Intel and AMD CPUs will trap to the host when the guest
is spinning on a spinlock, allowing the host to schedule in
something else.

This effectively means the host is taking care of spinlock
backoff for virtual machines. It also means that doing the
spinlock backoff in the guest anyway can lead to totally
unpredictable results, extremely large backoffs, and
performance regressions.

To prevent those problems, we limit the spinlock backoff
delay, when running in a virtual machine, to a small value.

Signed-off-by: Rik van Riel <riel@...hat.com>
---
 arch/x86/include/asm/processor.h |    2 ++
 arch/x86/kernel/cpu/hypervisor.c |    2 ++
 arch/x86/kernel/smp.c            |   21 +++++++++++++++++++--
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 888184b..4118fd8 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -997,6 +997,8 @@ extern bool cpu_has_amd_erratum(const int *);
 extern unsigned long arch_align_stack(unsigned long sp);
 extern void free_init_pages(char *what, unsigned long begin, unsigned long end);
 
+extern void init_guest_spinlock_delay(void);
+
 void default_idle(void);
 bool set_pm_idle_to_default(void);
 
diff --git a/arch/x86/kernel/cpu/hypervisor.c b/arch/x86/kernel/cpu/hypervisor.c
index a8f8fa9..4a53724 100644
--- a/arch/x86/kernel/cpu/hypervisor.c
+++ b/arch/x86/kernel/cpu/hypervisor.c
@@ -76,6 +76,8 @@ void __init init_hypervisor_platform(void)
 
 	init_hypervisor(&boot_cpu_data);
 
+	init_guest_spinlock_delay();
+
 	if (x86_hyper->init_platform)
 		x86_hyper->init_platform();
 }
diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index 64e33ef..fbc5ff3 100644
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -116,8 +116,25 @@ static bool smp_no_nmi_ipi = false;
 #define DELAY_SHIFT			8
 #define DELAY_FIXED_1			(1<<DELAY_SHIFT)
 #define MIN_SPINLOCK_DELAY		(1 * DELAY_FIXED_1)
-#define MAX_SPINLOCK_DELAY		(16000 * DELAY_FIXED_1)
+#define MAX_SPINLOCK_DELAY_NATIVE	(16000 * DELAY_FIXED_1)
+#define MAX_SPINLOCK_DELAY_GUEST	(16 * DELAY_FIXED_1)
 #define DELAY_HASH_SHIFT		6
+
+/*
+ * Modern Intel and AMD CPUs tell the hypervisor when a guest is
+ * spinning excessively on a spinlock. The hypervisor will then
+ * schedule something else, effectively taking care of the backoff
+ * for us. Doing our own backoff on top of the hypervisor's pause
+ * loop exit handling can lead to excessively long delays, and
+ * performance degradations. Limit the spinlock delay in virtual
+ * machines to a smaller value. Called from init_hypervisor_platform 
+ */
+static int __read_mostly max_spinlock_delay = MAX_SPINLOCK_DELAY_NATIVE;
+void __init init_guest_spinlock_delay(void)
+{
+	max_spinlock_delay = MAX_SPINLOCK_DELAY_GUEST;
+}
+
 struct delay_entry {
 	u32 hash;
 	u32 delay;
@@ -171,7 +188,7 @@ void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
 		}
 
 		/* Aggressively increase delay, to minimize lock accesses. */
-		if (delay < MAX_SPINLOCK_DELAY)
+		if (delay < max_spinlock_delay)
 			delay += DELAY_FIXED_1 / 7;
 
 		loops = (delay * waiters_ahead) >> DELAY_SHIFT;

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