Since I am resubmitting the nocb patches in this series (3 of them from this series) for the next merge window, I thought I'll replace this particular patch with just a deletion of the rcu_advance_cbs_nowake() call itself instead of bloating the code path with warnings and comments.
linux-next and many days of testing on my side are also looking good. Thoughts? Once I get any opinions, I'll change this patch to do the deletion. Also I am adding one other (trivial) patch to this series: https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git/commit/?h=nocb-7.0&id=84669d678b9cb28ff8774a3b6457186a4a187c75 Running overnight tests on all 4 patches now... thanks, - Joel On 1/1/2026 11:34 AM, Joel Fernandes wrote: > During callback overload, the NOCB code attempts an opportunistic > advancement via rcu_advance_cbs_nowake(). > > Analysis via tracing with 300,000 callbacks flooded shows this > optimization is likely dead code: > - 30 overload conditions triggered > - 0 advancements actually occurred > - 100% of time no advancement due to current GP not done. > > I also ran TREE05 and TREE08 for 2 hours and cannot trigger it. > > When callbacks overflow (exceed qhimark), they are waiting for a grace > period that hasn't completed yet. The optimization requires the GP to be > complete to advance callbacks, but the overload condition itself is > caused by callbacks piling up faster than GPs can complete. This creates > a logical contradiction where the advancement cannot happen. > > In *theory* this might be possible, the GP completed just in the nick of > time as we hit the overload, but this is just so rare that it can be > considered impossible when we cannot even hit it with synthetic callback > flooding even, it is a waste of cycles to even try to advance, let alone > be useful and is a maintenance burden complexity we don't need. > > I suggest deletion. However, add a WARN_ON_ONCE for a merge window or 2 > and delete it after out of extreme caution. > > Signed-off-by: Joel Fernandes <[email protected]> > --- > kernel/rcu/tree_nocb.h | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h > index 7e9d465c8ab1..d3e6a0e77210 100644 > --- a/kernel/rcu/tree_nocb.h > +++ b/kernel/rcu/tree_nocb.h > @@ -571,8 +571,20 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, > bool was_alldone, > if (j != rdp->nocb_gp_adv_time && > rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) && > rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq)) { > + long done_before = > rcu_segcblist_get_seglen(&rdp->cblist, RCU_DONE_TAIL); > + > rcu_advance_cbs_nowake(rdp->mynode, rdp); > rdp->nocb_gp_adv_time = j; > + > + /* > + * The advance_cbs call above is not useful. Under an > + * overload condition, nocb_gp_wait() is always waiting > + * for GP completion, due to this nothing can be moved > + * from WAIT to DONE, in the list. WARN if an > + * advancement happened (next step is deletion of > advance). > + */ > + WARN_ON_ONCE(rcu_segcblist_get_seglen(&rdp->cblist, > + RCU_DONE_TAIL) > done_before); > } > } >
