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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 , 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 Subject: Re: [RFC PATCH v4 3/4] hazptr: Implement Hazard Pointers Message-ID: References: <20251218014531.3793471-1-mathieu.desnoyers@efficios.com> <20251218014531.3793471-4-mathieu.desnoyers@efficios.com> <6c96dbb5-bffc-423f-bb6a-3072abb5f711@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <6c96dbb5-bffc-423f-bb6a-3072abb5f711@efficios.com> X-Rspam-User: X-Stat-Signature: ucd8qfrs38mp3w35wyai83zwhszodqck X-Rspamd-Queue-Id: D5C36C0014 X-Rspamd-Server: rspam04 X-HE-Tag: 1767890077-235660 X-HE-Meta: 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 lllwbZ8O FwkehBRnktTtWeE0TK5uGsrIn/tVqxAh2AFv3PGO5tDS08EhO3RjFNtJSed7vtBV9mFMV61pGjtl/y349hA8sJ384wQwSjxRAbietEV86TYUyVPpm4N4fIdinN7IbTLFT3LmnDq4cJUKqPEChjKFufB8acBeFZ0KNNV3x1oaAZM7irzQK7HlMapR7VZN8YKJ7rIzUvJP6+3xhu5bC6W9xgouf/8F9nOQvBZgLmuOz8DtwmIQut62QMvnOlYM1TQDQ2aaM4oB4U8dtudClRgSYQObwYZpnvbBOO5y15P1bFJsXu35P2jqam0HwhQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Le Fri, Dec 19, 2025 at 09:22:19AM -0500, Mathieu Desnoyers a écrit : > On 2025-12-18 19:43, Boqun Feng wrote: > > On Thu, Dec 18, 2025 at 12:35:18PM -0500, Mathieu Desnoyers wrote: > > [...] > > > > Could you utilize this[1] to see a > > > > comparison of the reader-side performance against RCU/SRCU? > > > > > > Good point ! 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 ? As far as I can tell, this skips clearing the state if the state is already cleared. Or am I even more sleep deprived than you? :o) Thanks. -- Frederic Weisbecker SUSE Labs