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McKenney" , linux-kernel@vger.kernel.org, Nicholas Piggin , Michael Ellerman , Greg Kroah-Hartman , Sebastian Andrzej Siewior , Will Deacon , Peter Zijlstra , Alan Stern , John Stultz , Neeraj Upadhyay , Linus Torvalds , Andrew Morton , Frederic Weisbecker , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Ingo Molnar , Waiman Long , Mark Rutland , Thomas Gleixner , Vlastimil Babka , maged.michael@gmail.com, Mateusz Guzik , Jonas Oberhauser , rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev References: <20251218014531.3793471-1-mathieu.desnoyers@efficios.com> <20251218014531.3793471-4-mathieu.desnoyers@efficios.com> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: YQBP288CA0033.CANP288.PROD.OUTLOOK.COM (2603:10b6:c01:9d::10) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: YT2PR01MB9175:EE_|TO1PPFF80C66F7A:EE_ X-MS-Office365-Filtering-Correlation-Id: 4b3b911b-1bd6-4aec-16d1-08de3f0a0621 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|7416014|1800799024; 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Let's see. >> >> On a AMD 2x EPYC 9654 96-Core Processor with 192 cores, >> hyperthreading disabled, >> CONFIG_PREEMPT=y, >> CONFIG_PREEMPT_RCU=y, >> CONFIG_PREEMPT_HAZPTR=y. >> >> scale_type ns >> ----------------------- >> hazptr-smp-mb 13.1 <- this implementation >> hazptr-barrier 11.5 <- replace smp_mb() on acquire with barrier(), requires IPIs on synchronize. >> hazptr-smp-mb-hlist 12.7 <- replace per-task hp context and per-cpu overflow lists by hlist. >> rcu 17.0 > > Hmm.. now looking back, how is it possible that hazptr is faster than > RCU on the reader-side? Because a grace period was happening and > triggered rcu_read_unlock_special()? This is actualy more interesting. So I could be entirely misreading the code, but, we have: rcu_flavor_sched_clock_irq(): [...] /* If GP is oldish, ask for help from rcu_read_unlock_special(). */ if (rcu_preempt_depth() > 0 && __this_cpu_read(rcu_data.core_needs_qs) && __this_cpu_read(rcu_data.cpu_no_qs.b.norm) && !t->rcu_read_unlock_special.b.need_qs && time_after(jiffies, rcu_state.gp_start + HZ)) t->rcu_read_unlock_special.b.need_qs = true; which means we set need_qs = true as a result from observing cpu_no_qs.b.norm == true. This is sufficient to trigger calls (plural) to rcu_read_unlock_special() from __rcu_read_unlock. But then if we look at rcu_preempt_deferred_qs_irqrestore() which we would expect to clear the rcu_read_unlock_special.b.need_qs state, we have this: special = t->rcu_read_unlock_special; if (!special.s && !rdp->cpu_no_qs.b.exp) { local_irq_restore(flags); return; } t->rcu_read_unlock_special.s = 0; which skips over clearing the state unless there is an expedited grace period required. So unless I'm missing something, we should _also_ clear that state when it's invoked after rcu_flavor_sched_clock_irq, so the next __rcu_read_unlock won't all call into rcu_read_unlock_special(). I'm adding a big warning about sleep deprivation and possibly misunderstanding the whole thing. What am I missing ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com