linux-kernel - [RFC PATCH] rcu: call kvm_check_and_clear_guest

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Date:   Fri, 16 Jul 2021 14:41:13 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>
Cc:     Suleiman Souhlal <suleiman@...gle.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: [RFC PATCH] rcu: call kvm_check_and_clear_guest_paused unconditionally

Do not call kvm_check_and_clear_guest_paused() only from the
stall branch (which requires an active grace period in the
first place), but instead handle PVCLOCK_GUEST_STOPPED as
early as possible.

pvclock_touch_watchdogs() touches various watchdogs, which
have different timeouts, so the earlier we handle stopped
VCPU the better (lockup watchdog does the same).

Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
---
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 677ee3d8671b..5dd6ea2ead0c 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -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))
-- 
2.32.0.402.g57bb445576-goog