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

[Sergey Senozhatsky]

On (21/05/22 00:56), 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.

Hello Paul,

I've noticed that this patch set didn't make it to Linus's tree.
Was it intentional?