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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 References: <20260101163417.1065705-1-joelagnelf@nvidia.com> <20260101163417.1065705-5-joelagnelf@nvidia.com> <20260109034930.GA1105379@joelbox2> Content-Language: en-US From: Joel Fernandes In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: MN2PR14CA0002.namprd14.prod.outlook.com (2603:10b6:208:23e::7) To SN7PR12MB8059.namprd12.prod.outlook.com (2603:10b6:806:32b::7) Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SN7PR12MB8059:EE_|CY8PR12MB7290:EE_ X-MS-Office365-Filtering-Correlation-Id: 8ac0c7a2-59e4-47f1-8fc9-08de4fc4e9c8 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|7416014|366016; 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>>>> } 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; >> } >> >> ? > > This else already means that rcu_segcblist_nextgp() returned false because there > is no pending callbacks. rcu_segcblist_empty() is different because it also test > done callbacks. > >> >> 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. > > So nocb_gp_wait (the rcuog kthreads) handle the pending callbacks, advancing > them throughout grace periods until they are moved to the done state. > > But once they are moved as done, the callbacks are the responsibility of > nocb_cb_wait() (the rcuo kthreads) and nocb_gp_wait() stops paying attention > to that rdp if there are no more pending callbacks. > > So with your initial patch, you still have rdp->nocb_gp_wake_attempt == true > even when there are only done callbacks. But without an appropriate wake-up > after subsequent enqueue, nocb_gp_wait() may ignore new callbacks, event though > rdp->nocb_gp_wake_attempt == true suggests otherwise. Ah, got it! I was clubbing the acting of waking up rcuog to that of both the rcuog and rcuop/s threads. Your suggestion, instead, is more accurate so I will do it that way instead. Thanks for the explanations! - Joel