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kvm: Note an RCU quiescent state on guest exit

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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@redhat.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240511020557.1198200-1-leobras@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Leonardo Bras 2024-05-10 23:05:56 -03:00 committed by Paolo Bonzini
parent d30d9ee94c
commit 593377036e
2 changed files with 13 additions and 3 deletions
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6
include/linux/context_tracking.h
View File

@ -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 */

10
include/linux/kvm_host.h
View File

@ -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();
}
}
/*