Le Thu, Jan 01, 2026 at 11:34:13AM -0500, Joel Fernandes a écrit :
> To be sure we have no rcog wake ups that were lost, add a warning
> to cover the case where the rdp is overloaded with callbacks but
> no wake up was attempted.
>
> Signed-off-by: Joel Fernandes <[email protected]>
> ---
> kernel/rcu/tree.c | 4 ++++
> kernel/rcu/tree.h | 1 +
> kernel/rcu/tree_nocb.h | 6 +++++-
> 3 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 293bbd9ac3f4..78c045a5ef03 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3767,6 +3767,10 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
> debug_rcu_head_unqueue(&rdp->barrier_head);
> rcu_barrier_trace(TPS("IRQNQ"), -1, rcu_state.barrier_sequence);
> }
> +#ifdef CONFIG_RCU_NOCB_CPU
> + if (wake_nocb)
> + rdp->nocb_gp_wake_attempt = true;
> +#endif
entrain only queues a callback if the list is non-empty. And if it's
non-empty, rdp->nocb_gp_wake_attempt should be true already.
> rcu_nocb_unlock(rdp);
> if (wake_nocb)
> wake_nocb_gp(rdp, false);
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 653fb4ba5852..74bd6a2a2f84 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -257,6 +257,7 @@ struct rcu_data {
> unsigned long nocb_gp_loops; /* # passes through wait code. */
> struct swait_queue_head nocb_gp_wq; /* For nocb kthreads to sleep on. */
> bool nocb_cb_sleep; /* Is the nocb CB thread asleep? */
> + bool nocb_gp_wake_attempt; /* Was a rcuog wakeup attempted? */
How about nocb_gp_handling ?
> struct task_struct *nocb_cb_kthread;
> struct list_head nocb_head_rdp; /*
> * Head of rcu_data list in wakeup
> chain,
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index daff2756cd90..7e9d465c8ab1 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -546,6 +546,7 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp,
> bool was_alldone,
> lazy_len = READ_ONCE(rdp->lazy_len);
> if (was_alldone) {
> rdp->qlen_last_fqs_check = len;
> + rdp->nocb_gp_wake_attempt = true;
> rcu_nocb_unlock(rdp);
> // Only lazy CBs in bypass list
> if (lazy_len && bypass_len == lazy_len) {
> @@ -563,7 +564,8 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp,
> bool was_alldone,
>
> return;
> } else if (len > rdp->qlen_last_fqs_check + qhimark) {
> - /* ... or if many callbacks queued. */
> + /* Callback overload condition. */
> + WARN_ON_ONCE(!rdp->nocb_gp_wake_attempt);
> rdp->qlen_last_fqs_check = len;
> j = jiffies;
> if (j != rdp->nocb_gp_adv_time &&
> @@ -688,6 +690,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
> bypass_ncbs > 2 * qhimark)) {
> flush_bypass = true;
> } else if (!bypass_ncbs && rcu_segcblist_empty(&rdp->cblist)) {
> + rdp->nocb_gp_wake_attempt = false;
This is when nocb_cb_wait() is done with callbacks but nocb_gp_wait() is done
with them sooner, when the grace period is done for all pending callbacks.
Something like this would perhaps be more accurate:
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index e6cd56603cad..52010cbeaa76 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -746,6 +746,8 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
needwait_gp = true;
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
TPS("NeedWaitGP"));
+ } else if (!rcu_cblist_n_cbs(&rdp->nocb_bypass)) {
+ rdp->nocb_gp_wake_attempt = false;
}
if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
needwake = rdp->nocb_cb_sleep;
> rcu_nocb_unlock_irqrestore(rdp, flags);
> continue; /* No callbacks here, try next. */
> }
> @@ -1254,6 +1257,7 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct
> shrink_control *sc)
> continue;
> }
> rcu_nocb_try_flush_bypass(rdp, jiffies);
> + rdp->nocb_gp_wake_attempt = true;
Same here, we should expect rdp->nocb_gp_wake_attempt to be already on since
there are lazy callbacks. That's a good opportunity to test the related
assertion
though.
Thanks.
> rcu_nocb_unlock_irqrestore(rdp, flags);
> wake_nocb_gp(rdp, false);
> sc->nr_to_scan -= _count;
> --
> 2.34.1
>
--
Frederic Weisbecker
SUSE Labs
