From 593377036e50de89132bc1222800174fde0780ec Mon Sep 17 00:00:00 2001 From: Leonardo Bras Date: Fri, 10 May 2024 23:05:56 -0300 Subject: 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 Acked-by: Paul E. McKenney Acked-by: Sean Christopherson Message-ID: <20240511020557.1198200-1-leobras@redhat.com> Signed-off-by: Paolo Bonzini --- 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 @@ -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 */ diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index b23c6d48392f..0d5125a3e31a 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -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(); + } } /* -- cgit v1.2.3