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

Paul E. McKenney
2024-04-01T15:52:23Z

On Thu, Mar 28, 2024 at 02:19:47PM -0300, Leonardo Bras wrote:
> 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>

Looks plausible to me!

Acked-by: Paul E. McKenney <paulmck@kernel.org>

Or let me know if you would rather these go through -rcu.

Thanx, Paul

> ---
> 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
>