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[34.228.114.98]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-5326208ea78sm56756741cf.24.2026.09.17.09.35.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 17 Sep 2026 09:35:27 -0700 (PDT) Date: Thu, 17 Sep 2026 16:35:19 +0000 Message-ID: From: Josef Bacik To: "Paul E. McKenney" , Frederic Weisbecker Cc: Neeraj Upadhyay , Joel Fernandes , Boqun Feng , Thomas Gleixner , Peter Zijlstra , Steven Rostedt , Masami Hiramatsu , Mark Rutland , Jiri Olsa , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , x86@kernel.org, Catalin Marinas , Will Deacon , Puranjay Mohan , Xu Kuohai , Andy Lutomirski , Josh Triplett , Uladzislau Rezki , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Juergen Gross , Luis Chamberlain , Ihor Solodrai , linux-kernel@vger.kernel.org, rcu@vger.kernel.org, linux-trace-kernel@vger.kernel.org, bpf@vger.kernel.org, linux-arm-kernel@lists.infradead.org, xen-devel@lists.xenproject.org Subject: Re: [PATCH RFC v3 03/13] rcu-tasks: Add a Tasks RCU implementation for reader-marked trampolines In-Reply-To: <8fea176a-0ca0-4a70-8cf9-8240a22359e4@paulmck-laptop> References: <20260915-b4-rcu-tasks-preempt-qs-v3-0-0ad30c4c5ee7@toxicpanda.com> <20260915-b4-rcu-tasks-preempt-qs-v3-3-0ad30c4c5ee7@toxicpanda.com> <91687748-0781-4133-baed-43cf82f35ff5@paulmck-laptop> <6afb0afe-e7b8-46b4-9fea-2c7cf4791044@paulmck-laptop> <8b73e1b3-0580-4ba9-b2e0-6363e429808d@paulmck-laptop> <8fea176a-0ca0-4a70-8cf9-8240a22359e4@paulmck-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260917_093530_099843_FA09809D X-CRM114-Status: GOOD ( 13.47 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, 17 Sep 2026 08:40:10 -0700, Paul E. McKenney wrote: > On Thu, Sep 17, 2026 at 02:14:57PM +0200, Frederic Weisbecker wrote: > > You're right. So we'll indeed need to reuse the deferred qs points here. > > Except this is getting a bit involved. > > Don't get me wrong, if Josef is happy to take this on, far be it from me > to stand in his way. But if not, we should be willing to treat this > optimization as a follow-on effort, whether by Josef or someone else. Follow-on works for me. For what it's worth 03/13 is already fairly close to what Frederic describes, just outside the core flavor: no task-list scan (the GP waits per CPU for a pass through __schedule() or an EQS), the irq-exit preemption path checks the interrupted IP and queues the task as a holdout before the switch, and the holdout is keyed on where the task was interrupted so nothing is needed from the trampoline tail. Moving that IP check into the tick / rcu_exp_handler() / deferred-QS paths and reusing the blocked-tasks list is something I'm happy to look at once this has settled. v4 will pick up Alexei's ask (reader emitted by the BPF JIT around the fentry and fexit regions rather than in the glue) and the idle-CPU hole Sashiko found. Thanks, Josef