Re: [RFC PATCH] rcu: call kvm_check_and_clear_guest_paused unconditionally

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (21/07/16 14:41), Sergey Senozhatsky wrote:
> @@ -657,6 +657,13 @@ static void check_cpu_stall(struct rcu_data *rdp)
>  	unsigned long js;
>  	struct rcu_node *rnp;
>  
> +	/*
> +	 * If a virtual machine is stopped by the host it can look to
> +	 * the watchdog like an RCU stall. Check to see if the host
> +	 * stopped the vm.
> +	 */
> +	kvm_check_and_clear_guest_paused();
> +
>  	lockdep_assert_irqs_disabled();
>  	if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) ||
>  	    !rcu_gp_in_progress())
> @@ -699,14 +706,6 @@ static void check_cpu_stall(struct rcu_data *rdp)
>  	    (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
>  	    cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
>  
> -		/*
> -		 * If a virtual machine is stopped by the host it can look to
> -		 * the watchdog like an RCU stall. Check to see if the host
> -		 * stopped the vm.
> -		 */
> -		if (kvm_check_and_clear_guest_paused())
> -			return;
> -
>  		/* We haven't checked in, so go dump stack. */
>  		print_cpu_stall(gps);
>  		if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
> @@ -717,14 +716,6 @@ static void check_cpu_stall(struct rcu_data *rdp)
>  		   ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
>  		   cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
>  
> -		/*
> -		 * If a virtual machine is stopped by the host it can look to
> -		 * the watchdog like an RCU stall. Check to see if the host
> -		 * stopped the vm.
> -		 */
> -		if (kvm_check_and_clear_guest_paused())
> -			return;
> -
>  		/* They had a few time units to dump stack, so complain. */
>  		print_other_cpu_stall(gs2, gps);
>  		if (READ_ONCE(rcu_cpu_stall_ftrace_dump))

This patch depends on
https://lore.kernel.org/lkml/20210716053405.1243239-1-senozhatsky@chromium.org/

If that x86/kvm patch lands, then we need to handle
PVCLOCK_GUEST_STOPPED in watchdogs.


In theory, this patch opens a small race window, if the VCPU gets preempted
after kvm_check_and_clear_guest_paused() (external interrupt, etc.)
But it's hard to tell how likely the problem is.