[RFC PATCH v1 2/2] rcu: Ignore RCU in nohz_full cpus if it was running a guest recently

[Leonardo Bras]

In current code, we can ignore the RCU request on a nohz_full cpu for up to
a second if it has interrupted idle or userspace tasks, since those are
quiescent states, and will probably return to it soon thus not requiring
to run a softirq or a rcuc thread.

Running a guest is also considered to be a quiescent state, and will
follow the same logic, so it makes sense to also ignore the RCU request in
this case.

This solves a latency issue of a latency-sensitive workload running on a
guest pinned in nohz_full cpu: if the guest goes out for any reason, and a
synchronize_rcu() is requested between guest exit and a timer interrupt,
then invoke_rcu_core() is called, and introduce latency due to either a
softirq, or a reschedule to run rcuc, if the host is a PREEMPT_RT kernel.

Suggested-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
Signed-off-by: Leonardo Bras <leobras@xxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 14 ++++++++++++++
 kernel/rcu/tree.c        |  4 +++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 36a8b5dbf5b5..16f3cf2e15df 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -5,20 +5,21 @@
  * or preemptible semantics.
  *
  * Copyright Red Hat, 2009
  * Copyright IBM Corporation, 2009
  *
  * Author: Ingo Molnar <mingo@xxxxxxx>
  *	   Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
  */
 
 #include "../locking/rtmutex_common.h"
+#include "linux/kvm_host.h"
 
 static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
 {
 	/*
 	 * In order to read the offloaded state of an rdp in a safe
 	 * and stable way and prevent from its value to be changed
 	 * under us, we must either hold the barrier mutex, the cpu
 	 * hotplug lock (read or write) or the nocb lock. Local
 	 * non-preemptible reads are also safe. NOCB kthreads and
 	 * timers have their own means of synchronization against the
@@ -1260,10 +1261,23 @@ static bool rcu_nohz_full_cpu(void)
 
 /*
  * Bind the RCU grace-period kthreads to the housekeeping CPU.
  */
 static void rcu_bind_gp_kthread(void)
 {
 	if (!tick_nohz_full_enabled())
 		return;
 	housekeeping_affine(current, HK_TYPE_RCU);
 }
+
+/*
+ * true if for this cpu guest exit is at most over a second ago,
+ * false otherwise
+ */
+static bool rcu_recent_guest_exit(void)
+{
+#ifdef CONFIG_KVM
+	return time_before(jiffies, guest_exit_last_time() + HZ);
+#else
+	return false;
+#endif
+}
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d9642dd06c25..e5ce00bf1898 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -148,20 +148,21 @@ static void rcu_report_qs_rnp(unsigned long mask, struct rcu_node *rnp,
 static struct task_struct *rcu_boost_task(struct rcu_node *rnp);
 static void invoke_rcu_core(void);
 static void rcu_report_exp_rdp(struct rcu_data *rdp);
 static void sync_sched_exp_online_cleanup(int cpu);
 static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node *rnp);
 static bool rcu_rdp_is_offloaded(struct rcu_data *rdp);
 static bool rcu_rdp_cpu_online(struct rcu_data *rdp);
 static bool rcu_init_invoked(void);
 static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf);
 static void rcu_init_new_rnp(struct rcu_node *rnp_leaf);
+static bool rcu_recent_guest_exit(void);
 
 /*
  * rcuc/rcub/rcuop kthread realtime priority. The "rcuop"
  * real-time priority(enabling/disabling) is controlled by
  * the extra CONFIG_RCU_NOCB_CPU_CB_BOOST configuration.
  */
 static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
 module_param(kthread_prio, int, 0444);
 
 /* Delay in jiffies for grace-period initialization delays, debug only. */
@@ -3931,21 +3932,22 @@ static int rcu_pending(int user)
 	lockdep_assert_irqs_disabled();
 
 	/* Check for CPU stalls, if enabled. */
 	check_cpu_stall(rdp);
 
 	/* Does this CPU need a deferred NOCB wakeup? */
 	if (rcu_nocb_need_deferred_wakeup(rdp, RCU_NOCB_WAKE))
 		return 1;
 
 	/* Is this a nohz_full CPU in userspace or idle?  (Ignore RCU if so.) */
-	if ((user || rcu_is_cpu_rrupt_from_idle()) && rcu_nohz_full_cpu())
+	if ((user || rcu_is_cpu_rrupt_from_idle() || rcu_recent_guest_exit()) &&
+	    rcu_nohz_full_cpu())
 		return 0;
 
 	/* Is the RCU core waiting for a quiescent state from this CPU? */
 	gp_in_progress = rcu_gp_in_progress();
 	if (rdp->core_needs_qs && !rdp->cpu_no_qs.b.norm && gp_in_progress)
 		return 1;
 
 	/* Does this CPU have callbacks ready to invoke? */
 	if (!rcu_rdp_is_offloaded(rdp) &&
 	    rcu_segcblist_ready_cbs(&rdp->cblist))
-- 
2.44.0