Commit d0e536d8 authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

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: Paul McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarRik van Riel <riel@redhat.com>
Tested-by: default avatarRik van Riel <riel@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent f70cd6b0

include/linux/context_tracking.h

+6 −2
Original line number Diff line number Diff line
@@ -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);

kernel/context_tracking.c

+38 −26
Original line number Diff line number Diff line
@@ -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);

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