Commit 59337703 authored by Leonardo Bras's avatar Leonardo Bras Committed by Paolo Bonzini
Browse files

kvm: Note an RCU quiescent state on guest exit 



As of today, KVM notes a quiescent state only in guest entry, which is good
as it avoids the guest being interrupted for current RCU operations.

While the guest vcpu runs, it can be interrupted by a timer IRQ that will
check for any RCU operations waiting for this CPU. In case there are any of
such, it invokes rcu_core() in order to sched-out the current thread and
note a quiescent state.

This occasional schedule work will introduce tens of microsseconds of
latency, which is really bad for vcpus running latency-sensitive
applications, such as real-time workloads.

So, note a quiescent state in guest exit, so the interrupted guests is able
to deal with any pending RCU operations before being required to invoke
rcu_core(), and thus avoid the overhead of related scheduler work.

Signed-off-by: default avatarLeonardo Bras <leobras@redhat.com>
Acked-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Acked-by: default avatarSean Christopherson <seanjc@google.com>
Message-ID: <20240511020557.1198200-1-leobras@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent d30d9ee9

include/linux/context_tracking.h

+4 −2
Original line number Diff line number Diff line
@@ -80,10 +80,12 @@ static __always_inline bool context_tracking_guest_enter(void) 
	return context_tracking_enabled_this_cpu();

}



static __always_inline void context_tracking_guest_exit(void)

static __always_inline bool context_tracking_guest_exit(void)

{

	if (context_tracking_enabled())

		__ct_user_exit(CONTEXT_GUEST);



	return context_tracking_enabled_this_cpu();

}



#define CT_WARN_ON(cond) WARN_ON(context_tracking_enabled() && (cond))
@@ -98,7 +100,7 @@ static inline void exception_exit(enum ctx_state prev_ctx) { } 
static inline int ct_state(void) { return -1; }

static inline int __ct_state(void) { return -1; }

static __always_inline bool context_tracking_guest_enter(void) { return false; }

static __always_inline void context_tracking_guest_exit(void) { }

static __always_inline bool context_tracking_guest_exit(void) { return false; }

#define CT_WARN_ON(cond) do { } while (0)

#endif /* !CONFIG_CONTEXT_TRACKING_USER */

include/linux/kvm_host.h

+9 −1
Original line number Diff line number Diff line
@@ -485,7 +485,15 @@ static __always_inline void guest_state_enter_irqoff(void) 
 */

static __always_inline void guest_context_exit_irqoff(void)

{

	context_tracking_guest_exit();

	/*

	 * Guest mode is treated as a quiescent state, see

	 * guest_context_enter_irqoff() for more details.

	 */

	if (!context_tracking_guest_exit()) {

		instrumentation_begin();

		rcu_virt_note_context_switch();

		instrumentation_end();

	}

}



/*