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 > to deal with any pending RCU operations before being required to invoke > rcu_core(), and thus avoid the overhead of related scheduler work. This does not properly fix the current problem, as RCU work might be scheduled after the VM exit, followed by a timer interrupt. Correct? > > Signed-off-by: Leonardo Bras <leobras@xxxxxxxxxx> > --- > > ps: A patch fixing this same issue was discussed in this thread: > https://lore.kernel.org/all/20240328171949.743211-1-leobras@xxxxxxxxxx/ > > 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 > > >
