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Mon, 19 Oct 2026 10:02:53 -0700 (PDT) Received: from localhost ([2a03:2880:32ff:47::]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-6b38a1b113dsm4084132a12.10.2026.10.19.10.02.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 19 Oct 2026 10:02:51 -0700 (PDT) From: Vlad Poenaru To: netdev@vger.kernel.org, Jamal Hadi Salim , Jiri Pirko , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman Cc: Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng , Waiman Long , Bart Van Assche , Davide Caratti , Breno Leitao , Nilay Shroff , linux-kernel@vger.kernel.org, Eric Dumazet Subject: [PATCH net-next 2/2] net/sched: Don't wait for RCU when unregistering the qdisc lockdep key Date: Mon, 19 Oct 2026 10:02:25 -0700 Message-ID: <20261019170225.729789-3-vlad.wing@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20261019170225.729789-1-vlad.wing@gmail.com> References: <20261019170225.729789-1-vlad.wing@gmail.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Since commit af0cb3fa3f9e ("net/sched: fix false lockdep warning on qdisc root lock") every qdisc except ingress, clsact and noqueue registers a dynamic lockdep key, and destroying it calls lockdep_unregister_key(), which waits for an expedited RCU grace period. Replacing an mq root destroys one qdisc per TX queue under RTNL, so a single "tc qdisc replace dev eth0 root mq" waits for that many grace periods, each of which IPIs every CPU that is not idle. The wait is not needed here: __qdisc_destroy() and qdisc_create() free the qdisc with qdisc_free_rcu() after unregistering, which already keeps the key's memory valid for a grace period. Use lockdep_unregister_key_nosync(), and free with kfree_rcu() in the one qdisc_alloc() error path that used kfree(). On an x86_64 KASAN + PROVE_LOCKING kernel (64 vCPU KVM guest, 64-queue netdevsim device, 214K lock chains) this removes 5.3 ms of grace-period wait from each of the 65 qdisc destroys in "tc qdisc replace dev eth0 root handle 1: mq". Suggested-by: Eric Dumazet Link: https://lore.kernel.org/all/CANn89iLvu_ijSV+FnrZ6qei_Ty2c9V-RjwLMk24+oOS91bNjaQ@mail.gmail.com/ Signed-off-by: Vlad Poenaru --- include/net/pkt_sched.h | 3 ++- net/sched/sch_generic.c | 2 +- 2 files changed, 3 insertions(+), 2 deletions(-) diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h index 90d3e7943b19a..1f4195dabaa57 100644 --- a/include/net/pkt_sched.h +++ b/include/net/pkt_sched.h @@ -323,6 +323,7 @@ static inline void qdisc_lock_init(struct Qdisc *sch, lockdep_set_class(&sch->q.lock, &sch->root_lock_key); } +/* Free @sch only after an RCU grace period, e.g. with qdisc_free_rcu(). */ static inline void qdisc_lock_uninit(struct Qdisc *sch, const struct Qdisc_ops *ops) { @@ -330,7 +331,7 @@ static inline void qdisc_lock_uninit(struct Qdisc *sch, ops == &noqueue_qdisc_ops) return; - lockdep_unregister_key(&sch->root_lock_key); + lockdep_unregister_key_nosync(&sch->root_lock_key); } #endif diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c index 6f6a6f0d5eb0d..28791d4fe35b1 100644 --- a/net/sched/sch_generic.c +++ b/net/sched/sch_generic.c @@ -1033,7 +1033,7 @@ struct Qdisc *qdisc_alloc(struct netdev_queue *dev_queue, return sch; errout1: qdisc_lock_uninit(sch, ops); - kfree(sch); + kfree_rcu(sch, rcu); errout: return ERR_PTR(err); } -- 2.53.0-Meta