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[PATCH 2/3] context_tracking: avoid irq_save/irq_restore on guest entry and exit

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  [PATCH 2/3] context_tracking: avoid irq_save/irq_restore on guest entry and exit Paolo Bonzini <pbonzini@redhat.com> - 2015-10-28 02:50 +0100
    Re: [PATCH 2/3] context_tracking: avoid irq_save/irq_restore on guest  entry and exit Andy Lutomirski <luto@amacapital.net> - 2015-10-28 06:30 +0100

#1257565 — [PATCH 2/3] context_tracking: avoid irq_save/irq_restore on guest entry and exit

guest_enter and guest_exit must be called with interrupts disabled,
since they take the vtime_seqlock with write_seq{lock,unlock}.
Therefore, it is not necessary to check for exceptions, nor to
save/restore the IRQ state, when context tracking functions are
called by guest_enter and guest_exit.

Split the body of context_tracking_entry and context_tracking_exit
out to __-prefixed functions, and use them from KVM.

Rik van Riel has measured this to speed up a tight vmentry/vmexit
loop by about 2%.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 include/linux/context_tracking.h |  8 +++--
 kernel/context_tracking.c        | 64 ++++++++++++++++++++++++----------------
 2 files changed, 44 insertions(+), 28 deletions(-)

diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index 6ef136ff0897..68b575afe5f5 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -10,6 +10,10 @@
 #ifdef CONFIG_CONTEXT_TRACKING
 extern void context_tracking_cpu_set(int cpu);
 
+/* Called with interrupts disabled.  */
+extern void __context_tracking_enter(enum ctx_state state);
+extern void __context_tracking_exit(enum ctx_state state);
+
 extern void context_tracking_enter(enum ctx_state state);
 extern void context_tracking_exit(enum ctx_state state);
 extern void context_tracking_user_enter(void);
@@ -88,13 +92,13 @@ static inline void guest_enter(void)
 		current->flags |= PF_VCPU;
 
 	if (context_tracking_is_enabled())
-		context_tracking_enter(CONTEXT_GUEST);
+		__context_tracking_enter(CONTEXT_GUEST);
 }
 
 static inline void guest_exit(void)
 {
 	if (context_tracking_is_enabled())
-		context_tracking_exit(CONTEXT_GUEST);
+		__context_tracking_exit(CONTEXT_GUEST);
 
 	if (vtime_accounting_enabled())
 		vtime_guest_exit(current);
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 6d4c6ce21275..d8560ee3bab7 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -58,27 +58,13 @@ static void context_tracking_recursion_exit(void)
  * instructions to execute won't use any RCU read side critical section
  * because this function sets RCU in extended quiescent state.
  */
-void context_tracking_enter(enum ctx_state state)
+void __context_tracking_enter(enum ctx_state state)
 {
-	unsigned long flags;
-
-	/*
-	 * Some contexts may involve an exception occuring in an irq,
-	 * leading to that nesting:
-	 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
-	 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
-	 * helpers are enough to protect RCU uses inside the exception. So
-	 * just return immediately if we detect we are in an IRQ.
-	 */
-	if (in_interrupt())
-		return;
-
 	/* Kernel threads aren't supposed to go to userspace */
 	WARN_ON_ONCE(!current->mm);
 
-	local_irq_save(flags);
 	if (!context_tracking_recursion_enter())
-		goto out_irq_restore;
+		return;
 
 	if ( __this_cpu_read(context_tracking.state) != state) {
 		if (__this_cpu_read(context_tracking.active)) {
@@ -111,7 +97,27 @@ void context_tracking_enter(enum ctx_state state)
 		__this_cpu_write(context_tracking.state, state);
 	}
 	context_tracking_recursion_exit();
-out_irq_restore:
+}
+NOKPROBE_SYMBOL(__context_tracking_enter);
+EXPORT_SYMBOL_GPL(__context_tracking_enter);
+
+void context_tracking_enter(enum ctx_state state)
+{
+	unsigned long flags;
+
+	/*
+	 * Some contexts may involve an exception occuring in an irq,
+	 * leading to that nesting:
+	 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
+	 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
+	 * helpers are enough to protect RCU uses inside the exception. So
+	 * just return immediately if we detect we are in an IRQ.
+	 */
+	if (in_interrupt())
+		return;
+
+	local_irq_save(flags);
+	__context_tracking_enter(state);
 	local_irq_restore(flags);
 }
 NOKPROBE_SYMBOL(context_tracking_enter);
@@ -135,16 +141,10 @@ NOKPROBE_SYMBOL(context_tracking_user_enter);
  * This call supports re-entrancy. This way it can be called from any exception
  * handler without needing to know if we came from userspace or not.
  */
-void context_tracking_exit(enum ctx_state state)
+void __context_tracking_exit(enum ctx_state state)
 {
-	unsigned long flags;
-
-	if (in_interrupt())
-		return;
-
-	local_irq_save(flags);
 	if (!context_tracking_recursion_enter())
-		goto out_irq_restore;
+		return;
 
 	if (__this_cpu_read(context_tracking.state) == state) {
 		if (__this_cpu_read(context_tracking.active)) {
@@ -161,7 +161,19 @@ void context_tracking_exit(enum ctx_state state)
 		__this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
 	}
 	context_tracking_recursion_exit();
-out_irq_restore:
+}
+NOKPROBE_SYMBOL(__context_tracking_exit);
+EXPORT_SYMBOL_GPL(__context_tracking_exit);
+
+void context_tracking_exit(enum ctx_state state)
+{
+	unsigned long flags;
+
+	if (in_interrupt())
+		return;
+
+	local_irq_save(flags);
+	__context_tracking_exit(state);
 	local_irq_restore(flags);
 }
 NOKPROBE_SYMBOL(context_tracking_exit);
-- 
2.5.0


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#1257667 — Re: [PATCH 2/3] context_tracking: avoid irq_save/irq_restore on guest entry and exit

On Tue, Oct 27, 2015 at 6:39 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> guest_enter and guest_exit must be called with interrupts disabled,
> since they take the vtime_seqlock with write_seq{lock,unlock}.
> Therefore, it is not necessary to check for exceptions, nor to
> save/restore the IRQ state, when context tracking functions are
> called by guest_enter and guest_exit.
>
> Split the body of context_tracking_entry and context_tracking_exit
> out to __-prefixed functions, and use them from KVM.
>
> Rik van Riel has measured this to speed up a tight vmentry/vmexit
> loop by about 2%.

Looks generally sensible.  I'm not familiar enough with the code to
call it reviewed-by while sitting on the airport shuttle.

--Andy
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