Re: [PATCHv2 1/2] rcu/tree: handle VM stoppage in stall detection

[Paul E. McKenney]

On Sat, May 22, 2021 at 12:56:23AM +0900, Sergey Senozhatsky wrote:
> Soft watchdog timer function checks if a virtual machine
> was suspended and hence what looks like a lockup in fact
> is a false positive.
> 
> This is what kvm_check_and_clear_guest_paused() does: it
> tests guest PVCLOCK_GUEST_STOPPED (which is set by the host)
> and if it's set then we need to touch all watchdogs and bail
> out.
> 
> Watchdog timer function runs from IRQ, so PVCLOCK_GUEST_STOPPED
> check works fine.
> 
> There is, however, one more watchdog that runs from IRQ, so
> watchdog timer fn races with it, and that watchdog is not aware
> of PVCLOCK_GUEST_STOPPED - RCU stall detector.
> 
> apic_timer_interrupt()
>  smp_apic_timer_interrupt()
>   hrtimer_interrupt()
>    __hrtimer_run_queues()
>     tick_sched_timer()
>      tick_sched_handle()
>       update_process_times()
>        rcu_sched_clock_irq()
> 
> This triggers RCU stalls on our devices during VM resume.
> 
> If tick_sched_handle()->rcu_sched_clock_irq() runs on a VCPU
> before watchdog_timer_fn()->kvm_check_and_clear_guest_paused()
> then there is nothing on this VCPU that touches watchdogs and
> RCU reads stale gp stall timestamp and new jiffies value, which
> makes it think that RCU has stalled.
> 
> Make RCU stall watchdog aware of PVCLOCK_GUEST_STOPPED and
> don't report RCU stalls when we resume the VM.
> 
> Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>

I have queued both for testing and further review, thank you!

							Thanx, Paul

> ---
> 
> v2: fixed powerpc build breakage
> 
>  kernel/rcu/tree_stall.h | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index d574e3bbd929..bc689911a81d 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -7,6 +7,8 @@
>   * Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
>   */
>  
> +#include <linux/kvm_para.h>
> +
>  //////////////////////////////////////////////////////////////////////////////
>  //
>  // Controlling CPU stall warnings, including delay calculation.
> @@ -698,6 +700,14 @@ 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))
> @@ -707,6 +717,14 @@ 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))
> -- 
> 2.31.1.818.g46aad6cb9e-goog
>