Re: [PATCH v1 10/11] rcu: Advance callbacks for expedited GP completion in rcu_core()

Le Tue, Jul 21, 2026 at 04:06:24PM +0100, Puranjay Mohan a écrit :

quoted

On Tue, Jul 21, 2026 at 3:35 PM Frederic Weisbecker [off-list ref] wrote:

quoted

Le Wed, Jun 24, 2026 at 06:23:52AM -0700, Puranjay Mohan a écrit :

quoted

Even when rcu_pending() triggers rcu_core(), the normal callback
advancement path through note_gp_changes() -> __note_gp_changes() bails
out when rdp->gp_seq == rnp->gp_seq (no normal GP change). Since
expedited GPs do not update rnp->gp_seq, rcu_advance_cbs() is never
called and callbacks remain stuck in RCU_WAIT_TAIL.

Add a direct callback advancement block in rcu_core() that checks for GP
completion via rcu_segcblist_nextgp() combined with
poll_state_synchronize_rcu_full(). When detected, trylock rnp and call
rcu_advance_cbs() to move completed callbacks to RCU_DONE_TAIL. Wake the
GP kthread if rcu_advance_cbs() requests a new grace period.

Uses trylock to avoid adding contention on rnp->lock. If the lock is
contended, callbacks will be advanced on the next tick.

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 kernel/rcu/tree.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b01d7bf6b57b1..f42e01ef479c4 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c

@@ -2891,6 +2891,23 @@ static __latent_entropy void rcu_core(void)
      /* Update RCU state based on any recent quiescent states. */
      rcu_check_quiescent_state(rdp);

+     /* Advance callbacks if an expedited GP has completed. */
+     if (!rcu_rdp_is_offloaded(rdp) && rcu_segcblist_is_enabled(&rdp->cblist)) {
+             struct rcu_gp_seq gp_state;
+
+             if (rcu_segcblist_nextgp(&rdp->cblist, &gp_state) &&
+                 poll_state_synchronize_rcu_full(&gp_state)) {
+                     guard(irqsave)();
+                     if (raw_spin_trylock_rcu_node(rnp)) {
+                             bool needwake = rcu_advance_cbs(rnp, rdp);
+
+                             raw_spin_unlock_rcu_node(rnp);
+                             if (needwake)
+                                     rcu_gp_kthread_wake();
+                     }
+             }
+     }

Should that go as an improvement to note_gp_changes() instead?

note_gp_changes() only reconciles rdp->gp_seq against rnp->gp_seq, and
the expedited path never advances rnp->gp_seq. So the gap this closes
is exactly rdp->gp_seq == rnp->gp_seq, where note_gp_changes() and
__note_gp_changes() both short-circuit, the expedited completion isn't
visible there at all. It's detected from the cblist's stored gp_seq
(rcu_segcblist_nextgp()) confirmed with
poll_state_synchronize_rcu_full(), so hosting it in note_gp_changes()
would mean running that in the lockless preamble for every caller,
including the off-tick call_rcu_core() path. In rcu_core() it's
already gated by rcu_pending(), which does the barrier-free detection.

Let's take a step back. note_gp_changes() is for the CPU to ackowledge
a grace period change, either start or completion, and react upon with:

_ Making the callback progress through the state machine if a grace period
  has changed.

_ Starting to chase quiescent states.

And now callback advancing/acceleration don't even refer anymore to the
leaf node state but to the global one. So why not proceed with that
logic?

Also other callers of note_gp_changes() may want to benefit from expedited
grace periods as well.

Would the following (untested) work?

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ff6601411a89..96bf7fe03be8 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c

@@ -1271,27 +1271,29 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 {
        bool ret = false;
        bool need_qs;
+       struct rcu_gp_seq gp_state;
        const bool offloaded = rcu_rdp_is_offloaded(rdp);

        raw_lockdep_assert_held_rcu_node(rnp);

-       if (rdp->gp_seq == rnp->gp_seq)
-               return false; /* Nothing to do. */
-
        /* Handle the ends of any preceding grace periods first. */
-       if (rcu_seq_completed_gp(rdp->gp_seq, rnp->gp_seq) ||
+       if ((rcu_segcblist_nextgp(&rdp->cblist, &gp_state) &&
+           poll_state_synchronize_rcu_full_unordered(&gp_state)) ||
            unlikely(rdp->gpwrap)) {
                if (!offloaded)
                        ret = rcu_advance_cbs(rnp, rdp); /* Advance CBs. */
                rdp->core_needs_qs = false;
                trace_rcu_grace_period(rcu_state.name, rdp->gp_seq, TPS("cpuend"));
-       } else {
+       } else if (rdp->gp_seq != rnp->gp_seq) {
                if (!offloaded)
                        ret = rcu_accelerate_cbs(rnp, rdp); /* Recent CBs. */
                if (rdp->core_needs_qs)
                        rdp->core_needs_qs = !!(rnp->qsmask & rdp->grpmask);
        }

+       if (rdp->gp_seq == rnp->gp_seq)
+               return ret; /* Nothing else to do. */
+
        /* Now handle the beginnings of any new-to-this-CPU grace periods. */
        if (rcu_seq_new_gp(rdp->gp_seq, rnp->gp_seq) ||
            unlikely(rdp->gpwrap)) {

@@ -1316,6 +1318,27 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
        return ret;
 }

+static bool need_note_gp_changes(struct rcu_data *rdp)
+{
+       struct rcu_gp_seq gp_state;
+       struct rcu_node *rnp = rdp->mynode;
+
+       /* Need to chase QS or accelerate? */
+       if (rdp->gp_seq != rcu_seq_current(&rnp->gp_seq))
+               return true;
+
+       /* Waited upon GP has ended, need to advance CBs ? */
+       if (rcu_segcblist_nextgp(&rdp->cblist, &gp_state) &&
+           poll_state_synchronize_rcu_full_unordered(&gp_state))
+               return true;
+
+       /* Wrapped? */
+       if (unlikely(READ_ONCE(rdp->gpwrap)))
+               return true;
+
+       return false;
+}
+
 static void note_gp_changes(struct rcu_data *rdp)
 {
        unsigned long flags;

@@ -1324,8 +1347,7 @@ static void note_gp_changes(struct rcu_data *rdp)

        local_irq_save(flags);
        rnp = rdp->mynode;
-       if ((rdp->gp_seq == rcu_seq_current(&rnp->gp_seq) &&
-            !unlikely(READ_ONCE(rdp->gpwrap))) || /* w/out lock. */
+       if (!need_note_gp_changes(rdp) || /* w/out lock. */
            !raw_spin_trylock_rcu_node(rnp)) { /* irqs already off, so later. */
                local_irq_restore(flags);
                return;

@@ -2888,23 +2910,6 @@ static __latent_entropy void rcu_core(void)
        /* Update RCU state based on any recent quiescent states. */
        rcu_check_quiescent_state(rdp);

-       /* Advance callbacks if an expedited GP has completed. */
-       if (!rcu_rdp_is_offloaded(rdp) && rcu_segcblist_is_enabled(&rdp->cblist)) {
-               struct rcu_gp_seq gp_state;
-
-               if (rcu_segcblist_nextgp(&rdp->cblist, &gp_state) &&
-                   poll_state_synchronize_rcu_full(&gp_state)) {
-                       guard(irqsave)();
-                       if (raw_spin_trylock_rcu_node(rnp)) {
-                               bool needwake = rcu_advance_cbs(rnp, rdp);
-
-                               raw_spin_unlock_rcu_node(rnp);
-                               if (needwake)
-                                       rcu_gp_kthread_wake();
-                       }
-               }
-       }
-
        /* No grace period and unregistered callbacks? */
        if (!rcu_gp_in_progress() &&
            rcu_segcblist_is_enabled(&rdp->cblist) && !rcu_rdp_is_offloaded(rdp)) {

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 01a1b2985abd..6b9b058d138e 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h

@@ -517,6 +517,7 @@ static void rcu_nocb_unlock(struct rcu_data *rdp);
 static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
                                       unsigned long flags);
 static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp);
+static bool poll_state_synchronize_rcu_full_unordered(struct rcu_gp_seq *gsp);
 #ifdef CONFIG_RCU_NOCB_CPU
 static void __init rcu_organize_nocb_kthreads(void);