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SubjectRe: [RFC PATCH 1/1] kvm: Note an RCU quiescent state on guest exit
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On Fri, May 10, 2024 at 11:05:56PM -0300, Leonardo Bras wrote:
> 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

s/guests/guest

> 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>
> ---
>
> 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
>


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