linux-kernel - [RFC PATCH 1/1] kvm: Note an RCU quiescent state on guest exit

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Date: Fri, 10 May 2024 23:05:56 -0300
From: Leonardo Bras <leobras@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Marcelo Tosatti <mtosatti@...hat.com>
Cc: Leonardo Bras <leobras@...hat.com>,
	linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: [RFC PATCH 1/1] 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: Leonardo Bras <leobras@...hat.com>
---

ps: A patch fixing this same issue was discussed in this thread:
https://lore.kernel.org/all/20240328171949.743211-1-leobras@redhat.com/

Also, this can be paired with a new RCU option (rcutree.nocb_patience_delay)
to avoid having invoke_rcu() being called on grace-periods starting between
guest exit and the timer IRQ. This RCU option is being discussed in a
sub-thread of this message:
https://lore.kernel.org/all/5fd66909-1250-4a91-aa71-93cb36ed4ad5@paulmck-laptop/


 include/linux/context_tracking.h |  6 ++++--
 include/linux/kvm_host.h         | 10 +++++++++-
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index 6e76b9dba00e..8a78fabeafc3 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -73,39 +73,41 @@ static inline void exception_exit(enum ctx_state prev_ctx)
 }
 
 static __always_inline bool context_tracking_guest_enter(void)
 {
 	if (context_tracking_enabled())
 		__ct_user_enter(CONTEXT_GUEST);
 
 	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))
 
 #else
 static inline void user_enter(void) { }
 static inline void user_exit(void) { }
 static inline void user_enter_irqoff(void) { }
 static inline void user_exit_irqoff(void) { }
 static inline int exception_enter(void) { return 0; }
 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 */
 
 #ifdef CONFIG_CONTEXT_TRACKING_USER_FORCE
 extern void context_tracking_init(void);
 #else
 static inline void context_tracking_init(void) { }
 #endif /* CONFIG_CONTEXT_TRACKING_USER_FORCE */
 
 #ifdef CONFIG_CONTEXT_TRACKING_IDLE
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 48f31dcd318a..e37724c44843 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -480,21 +480,29 @@ static __always_inline void guest_state_enter_irqoff(void)
 /*
  * Exit guest context and exit an RCU extended quiescent state.
  *
  * Between guest_context_enter_irqoff() and guest_context_exit_irqoff() it is
  * unsafe to use any code which may directly or indirectly use RCU, tracing
  * (including IRQ flag tracing), or lockdep. All code in this period must be
  * non-instrumentable.
  */
 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();
+	}
 }
 
 /*
  * Stop accounting time towards a guest.
  * Must be called after exiting guest context.
  */
 static __always_inline void guest_timing_exit_irqoff(void)
 {
 	instrumentation_begin();
 	/* Flush the guest cputime we spent on the guest */
-- 
2.45.0