rcu/tree: Handle VM stoppage in stall detection

The 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@chromium.org>
Signed-off-by: Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_stall.h

@@ -7,6 +7,8 @@
  * Author: Paul E. McKenney <paulmck@linux.ibm.com>
  */
 
+#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))