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Date: Sun, 12 May 2024 18:44:23 -0300
From: Marcelo Tosatti <mtosatti@redhat.com>
To: Leonardo Bras <leobras@redhat.com>
Cc: Frederic Weisbecker <frederic@kernel.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [RFC PATCH 1/1] kvm: Note an RCU quiescent state on guest exit
Message-ID: <ZkE4N1X0wglygt75@tpad>
References: <20240511020557.1198200-1-leobras@redhat.com>
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In-Reply-To: <20240511020557.1198200-1-leobras@redhat.com>

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@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
> 
> 
>