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McKenney" , Boqun Feng , rcu@vger.kernel.org, Frederic Weisbecker , Neeraj Upadhyay , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Joel Fernandes Subject: [PATCH -next v2 1/4] rcu/nocb: Remove unnecessary WakeOvfIsDeferred wake path Date: Wed, 14 Jan 2026 12:31:51 -0500 Message-Id: <20260114173154.1701056-2-joelagnelf@nvidia.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260114173154.1701056-1-joelagnelf@nvidia.com> References: <20260114173154.1701056-1-joelagnelf@nvidia.com> Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: MN2PR03CA0021.namprd03.prod.outlook.com (2603:10b6:208:23a::26) To PH7PR12MB8056.namprd12.prod.outlook.com (2603:10b6:510:269::21) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH7PR12MB8056:EE_|DM6PR12MB4202:EE_ X-MS-Office365-Filtering-Correlation-Id: 85df0f9c-d7a2-43d1-a13a-08de5392dd5d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014|7416014; 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charset="utf-8" The WakeOvfIsDeferred code path in __call_rcu_nocb_wake() attempts to wake rcuog when the callback count exceeds qhimark and callbacks aren't done with their GP (newly queued or awaiting GP). However, a lot of testing proves this wake is always redundant or useless. In the flooding case, rcuog is always waiting for a GP to finish. So waking up the rcuog thread is pointless. The timer wakeup adds overhead, rcuog simply wakes up and goes back to sleep achieving nothing. This path also adds a full memory barrier, and additional timer expiry modifications unnecessarily. The root cause is that WakeOvfIsDeferred fires when !rcu_segcblist_ready_cbs() (GP not complete), but waking rcuog cannot accelerate GP completion. This commit therefore removes this path. Tested with rcutorture scenarios: TREE01, TREE05, TREE08 (all NOCB configurations) - all pass. Also stress tested using a kernel module that floods call_rcu() to trigger the overload conditions and made the observations confirming the findings. Reviewed-by: Frederic Weisbecker Signed-off-by: Joel Fernandes --- kernel/rcu/tree.c | 2 +- kernel/rcu/tree.h | 3 +-- kernel/rcu/tree_nocb.h | 49 ++++++++++++++---------------------------- 3 files changed, 18 insertions(+), 36 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 293bbd9ac3f4..2921ffb19939 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3769,7 +3769,7 @@ static void rcu_barrier_entrain(struct rcu_data *rdp) } rcu_nocb_unlock(rdp); if (wake_nocb) - wake_nocb_gp(rdp, false); + wake_nocb_gp(rdp); smp_store_release(&rdp->barrier_seq_snap, gseq); } =20 diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 2265b9c2906e..7dfc57e9adb1 100644 --- a/kernel/rcu/tree.h +++ b/kernel/rcu/tree.h @@ -301,7 +301,6 @@ struct rcu_data { #define RCU_NOCB_WAKE_BYPASS 1 #define RCU_NOCB_WAKE_LAZY 2 #define RCU_NOCB_WAKE 3 -#define RCU_NOCB_WAKE_FORCE 4 =20 #define RCU_JIFFIES_TILL_FORCE_QS (1 + (HZ > 250) + (HZ > 500)) /* For jiffies_till_first_fqs and */ @@ -500,7 +499,7 @@ static void zero_cpu_stall_ticks(struct rcu_data *rdp); static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp); static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq); static void rcu_init_one_nocb(struct rcu_node *rnp); -static bool wake_nocb_gp(struct rcu_data *rdp, bool force); +static bool wake_nocb_gp(struct rcu_data *rdp); static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *r= hp, unsigned long j, bool lazy); static void call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *head, diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index e6cd56603cad..f525e4f7985b 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -192,7 +192,7 @@ static void rcu_init_one_nocb(struct rcu_node *rnp) =20 static bool __wake_nocb_gp(struct rcu_data *rdp_gp, struct rcu_data *rdp, - bool force, unsigned long flags) + unsigned long flags) __releases(rdp_gp->nocb_gp_lock) { bool needwake =3D false; @@ -209,7 +209,7 @@ static bool __wake_nocb_gp(struct rcu_data *rdp_gp, timer_delete(&rdp_gp->nocb_timer); } =20 - if (force || READ_ONCE(rdp_gp->nocb_gp_sleep)) { + if (READ_ONCE(rdp_gp->nocb_gp_sleep)) { WRITE_ONCE(rdp_gp->nocb_gp_sleep, false); needwake =3D true; } @@ -225,13 +225,13 @@ static bool __wake_nocb_gp(struct rcu_data *rdp_gp, /* * Kick the GP kthread for this NOCB group. */ -static bool wake_nocb_gp(struct rcu_data *rdp, bool force) +static bool wake_nocb_gp(struct rcu_data *rdp) { unsigned long flags; struct rcu_data *rdp_gp =3D rdp->nocb_gp_rdp; =20 raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags); - return __wake_nocb_gp(rdp_gp, rdp, force, flags); + return __wake_nocb_gp(rdp_gp, rdp, flags); } =20 #ifdef CONFIG_RCU_LAZY @@ -518,10 +518,8 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, = struct rcu_head *rhp, } =20 /* - * Awaken the no-CBs grace-period kthread if needed, either due to it - * legitimately being asleep or due to overload conditions. - * - * If warranted, also wake up the kthread servicing this CPUs queues. + * Awaken the no-CBs grace-period kthread if needed due to it legitimately + * being asleep. */ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone, unsigned long flags) @@ -533,7 +531,6 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, = bool was_alldone, long lazy_len; long len; struct task_struct *t; - struct rcu_data *rdp_gp =3D rdp->nocb_gp_rdp; =20 // If we are being polled or there is no kthread, just leave. t =3D READ_ONCE(rdp->nocb_gp_kthread); @@ -549,22 +546,22 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp= , bool was_alldone, lazy_len =3D READ_ONCE(rdp->lazy_len); if (was_alldone) { rdp->qlen_last_fqs_check =3D len; + rcu_nocb_unlock(rdp); // Only lazy CBs in bypass list if (lazy_len && bypass_len =3D=3D lazy_len) { - rcu_nocb_unlock(rdp); wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY, TPS("WakeLazy")); } else if (!irqs_disabled_flags(flags)) { /* ... if queue was empty ... */ - rcu_nocb_unlock(rdp); - wake_nocb_gp(rdp, false); + wake_nocb_gp(rdp); trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeEmpty")); } else { - rcu_nocb_unlock(rdp); wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE, TPS("WakeEmptyIsDeferred")); } + + return; } else if (len > rdp->qlen_last_fqs_check + qhimark) { /* ... or if many callbacks queued. */ rdp->qlen_last_fqs_check =3D len; @@ -575,21 +572,10 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp= , bool was_alldone, rcu_advance_cbs_nowake(rdp->mynode, rdp); rdp->nocb_gp_adv_time =3D j; } - smp_mb(); /* Enqueue before timer_pending(). */ - if ((rdp->nocb_cb_sleep || - !rcu_segcblist_ready_cbs(&rdp->cblist)) && - !timer_pending(&rdp_gp->nocb_timer)) { - rcu_nocb_unlock(rdp); - wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE, - TPS("WakeOvfIsDeferred")); - } else { - rcu_nocb_unlock(rdp); - trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot")); - } - } else { - rcu_nocb_unlock(rdp); - trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot")); } + + rcu_nocb_unlock(rdp); + trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot")); } =20 static void call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *head, @@ -966,7 +952,6 @@ static bool do_nocb_deferred_wakeup_common(struct rcu_d= ata *rdp_gp, unsigned long flags) __releases(rdp_gp->nocb_gp_lock) { - int ndw; int ret; =20 if (!rcu_nocb_need_deferred_wakeup(rdp_gp, level)) { @@ -974,8 +959,7 @@ static bool do_nocb_deferred_wakeup_common(struct rcu_d= ata *rdp_gp, return false; } =20 - ndw =3D rdp_gp->nocb_defer_wakeup; - ret =3D __wake_nocb_gp(rdp_gp, rdp, ndw =3D=3D RCU_NOCB_WAKE_FORCE, flags= ); + ret =3D __wake_nocb_gp(rdp_gp, rdp, flags); trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DeferredWake")); =20 return ret; @@ -991,7 +975,6 @@ static void do_nocb_deferred_wakeup_timer(struct timer_= list *t) trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Timer")); =20 raw_spin_lock_irqsave(&rdp->nocb_gp_lock, flags); - smp_mb__after_spinlock(); /* Timer expire before wakeup. */ do_nocb_deferred_wakeup_common(rdp, rdp, RCU_NOCB_WAKE_BYPASS, flags); } =20 @@ -1272,7 +1255,7 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct = shrink_control *sc) } rcu_nocb_try_flush_bypass(rdp, jiffies); rcu_nocb_unlock_irqrestore(rdp, flags); - wake_nocb_gp(rdp, false); + wake_nocb_gp(rdp); sc->nr_to_scan -=3D _count; count +=3D _count; if (sc->nr_to_scan <=3D 0) @@ -1657,7 +1640,7 @@ static void rcu_init_one_nocb(struct rcu_node *rnp) { } =20 -static bool wake_nocb_gp(struct rcu_data *rdp, bool force) +static bool wake_nocb_gp(struct rcu_data *rdp) { return false; 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charset="utf-8" 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. [applied Frederic's adjustment to clearing of nocb_gp_handling flag] Signed-off-by: Joel Fernandes --- kernel/rcu/tree.c | 4 ++++ kernel/rcu/tree.h | 1 + kernel/rcu/tree_nocb.h | 11 ++++++++++- 3 files changed, 15 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 2921ffb19939..958b61be87ea 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 + /* wake_nocb implies all CBs queued before were bypass/lazy. */ + WARN_ON_ONCE(wake_nocb && !rdp->nocb_gp_handling); +#endif rcu_nocb_unlock(rdp); if (wake_nocb) wake_nocb_gp(rdp); diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h index 7dfc57e9adb1..af1d065e3215 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_handling; /* Is rcuog handling this rdp? */ 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 f525e4f7985b..acca24670a8c 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 =3D READ_ONCE(rdp->lazy_len); if (was_alldone) { rdp->qlen_last_fqs_check =3D len; + rdp->nocb_gp_handling =3D true; rcu_nocb_unlock(rdp); // Only lazy CBs in bypass list if (lazy_len && bypass_len =3D=3D lazy_len) { @@ -563,7 +564,8 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, = bool was_alldone, =20 return; } else if (len > rdp->qlen_last_fqs_check + qhimark) { - /* ... or if many callbacks queued. */ + /* Callback overload condition. */ + WARN_ON_ONCE(!rdp->nocb_gp_handling); 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charset="utf-8" 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 --- 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 acca24670a8c..702ede003dce 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 !=3D 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 =3D rcu_segcblist_get_seglen(&rdp->cblist, RCU_DONE_TA= IL); + rcu_advance_cbs_nowake(rdp->mynode, rdp); rdp->nocb_gp_adv_time =3D 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. 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charset="utf-8" The pattern of checking nocb_defer_wakeup and deleting the timer is duplicated in __wake_nocb_gp() and nocb_gp_wait(). Extract this into a common helper function nocb_defer_wakeup_cancel(). This removes code duplication and makes it easier to maintain. Signed-off-by: Joel Fernandes Reviewed-by: Frederic Weisbecker --- kernel/rcu/tree_nocb.h | 19 +++++++++++-------- 1 file changed, 11 insertions(+), 8 deletions(-) diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index 702ede003dce..df49c2fa79c5 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -190,6 +190,15 @@ static void rcu_init_one_nocb(struct rcu_node *rnp) init_swait_queue_head(&rnp->nocb_gp_wq[1]); } =20 +/* Clear any pending deferred wakeup timer (nocb_gp_lock must be held). */ +static void nocb_defer_wakeup_cancel(struct rcu_data *rdp_gp) +{ + if (rdp_gp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) { + WRITE_ONCE(rdp_gp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT); + timer_delete(&rdp_gp->nocb_timer); + } +} + static bool __wake_nocb_gp(struct rcu_data *rdp_gp, struct rcu_data *rdp, unsigned long flags) @@ -204,10 +213,7 @@ static bool __wake_nocb_gp(struct rcu_data *rdp_gp, return false; } =20 - if (rdp_gp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) { - WRITE_ONCE(rdp_gp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT); - timer_delete(&rdp_gp->nocb_timer); - } + nocb_defer_wakeup_cancel(rdp_gp); =20 if (READ_ONCE(rdp_gp->nocb_gp_sleep)) { WRITE_ONCE(rdp_gp->nocb_gp_sleep, false); @@ -820,10 +826,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp) if (rdp_toggling) my_rdp->nocb_toggling_rdp =3D NULL; =20 - if (my_rdp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) { - WRITE_ONCE(my_rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT); - timer_delete(&my_rdp->nocb_timer); - } + nocb_defer_wakeup_cancel(my_rdp); WRITE_ONCE(my_rdp->nocb_gp_sleep, true); raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags); } else { --=20 2.34.1