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McKenney" , Boqun Feng , rcu@vger.kernel.org, Neeraj Upadhyay , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Shuah Khan , linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH -next 4/8] rcu/nocb: Add warning if no rcuog wake up attempt happened during overload Message-ID: <20260109034930.GA1105379@joelbox2> References: <20260101163417.1065705-1-joelagnelf@nvidia.com> <20260101163417.1065705-5-joelagnelf@nvidia.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Hi Frederic, On Thu, Jan 08, 2026 at 06:22:45PM +0100, Frederic Weisbecker wrote: > 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 > > --- > > 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. This is true, we don't need to track this wake up. I will replace it with a WARN. > > 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 ? This is a better name indeed, considering that we also track this for deferred wakeups of the GP thread. > > 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; > } Hmm, I am trying to understand why this suggestion is better than what I already have. It is one extra line and adds another conditional. Also shouldn't it be: } else if (!rcu_cblist_n_cbs(&rdp->nocb_bypass) && rcu_segcblist_empty(&rdp->cblist)) { rdp->nocb_gp_wake_attempt = false; } ? My goal was to mark wake_attempt as false when ALL callbacks on the rdp were drained. IOW, the GP thread is done with the rdp. > 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. Good point! I will turn it into a WARN. Btw, I have more patches coming to simplify nocb_gp_wait()... it is quite long :) thanks, - Joel