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

Leonardo Bras
2024-03-28T17:20:09Z

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@redhat.com>
Signed-off-by: Leonardo Bras <leobras@redhat.com>
---
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@elte.hu>
* Paul E. McKenney <paulmck@linux.ibm.com>
*/

#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