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 <[email protected]> Signed-off-by: Leonardo Bras <[email protected]> --- 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 <[email protected]> * Paul E. McKenney <[email protected]> */ #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
