From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7C02EC982C3 for ; Wed, 16 Sep 2026 15:23:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To: Content-Transfer-Encoding:Content-Type:MIME-Version:References:Message-ID: Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=gUBqB4Fqzqc99qDiZRC6rn2LnblCueCLqVsZm7n7TfU=; b=k760BjOmN3NTDH+QoYy/yZgskE gSz1E5izMR2C2ZCuxpRy5pr7jUjNTjbu7fR6I619cS3Zxd4YTXp/UX74rsj1Y/fv6K37atGVrc0ep TKmyyE4FylY+HmDZySKCixOsbzL7jPOoKHcfVTVvhGO6xM9FqGkjZTL5UD+UOJDXyDxBauC2uHbIh Sh6j4udc3EgKkPCA3VbEjCGAXaSQP7VZYXfO6+DCDvLisLIHd6rrCOEMOZjoY3BeHLqo5GZS51IdG 2ArBOFGqzldLp6EBC0WENNOnrrldblAmUJ2aNKhDtz/Clw0uuw0E+6QtWL3i+kBxLgRf2erCKNW5C PV8ljEMw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1x6rU0-00000009ZR7-1BJM; Wed, 16 Sep 2026 15:23:36 +0000 Received: from sea.source.kernel.org ([172.234.252.31]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1x6rTy-00000009ZQr-106B for linux-arm-kernel@lists.infradead.org; Wed, 16 Sep 2026 15:23:34 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 0310C43670; Wed, 16 Sep 2026 15:23:34 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 341851F000FF; Wed, 16 Sep 2026 15:23:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789572213; bh=gUBqB4Fqzqc99qDiZRC6rn2LnblCueCLqVsZm7n7TfU=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=Zms7zbQMXsV+X5ghNSF6YjMl6cJUjNxOkr2rrVhGM0XFYBR9/EjTqpLFG0bk/C0J+ F29El3ZivUeVL99Zn3f4rNP0UQ71wVGtSaM4Wr/EMConrjhwND2seSG6X9LdI8PKKt wBlIcZKq1r/aZ74++NVwbVmjyfyz5z1nYuTWwX+kD2jkBcYRdj7Mi6GJnPNsh85OIT Wt/c6bCxiBrWxpCWyEuLB8YAvC09SN9sOOK19ICTHSvz2x5HNnAUJXlBqgbo7+D6OZ nx0j/Hi/XmglqjOXV6BadfdwTE/hibURx5tCXMkuCpoOvvLMWTRan6RTc2WQiI6iCr 47EQKZi3eSznQ== Date: Wed, 16 Sep 2026 17:23:31 +0200 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: Josef Bacik , 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 Message-ID: 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> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <6afb0afe-e7b8-46b4-9fea-2c7cf4791044@paulmck-laptop> 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 Le Wed, Sep 16, 2026 at 07:55:02AM -0700, Paul E. McKenney a écrit : > On Wed, Sep 16, 2026 at 04:47:29PM +0200, Frederic Weisbecker wrote: > > Le Wed, Sep 16, 2026 at 04:35:50PM +0200, Frederic Weisbecker a écrit : > > > Le Wed, Sep 16, 2026 at 07:26:35AM -0700, Paul E. McKenney a écrit : > > > > On Wed, Sep 16, 2026 at 02:40:20PM +0200, Frederic Weisbecker wrote: > > > > > Le Tue, Sep 15, 2026 at 04:56:41PM -0700, Paul E. McKenney a écrit : > > > > > > > Alternatively the approach could be generalized to vanilla RCU, it could be > > > > > > > possible to define a .text.rcu_no_qs section within which code running is > > > > > > > considered as an RCU reader (with a pause while on the explicit RCU tasks > > > > > > > section). It would be forbidden to voluntary sleep inside > > > > > > > and to put explicit preemption points (CONFIG_PROVE_RCU could report misuses). > > > > > > > > > > > > > > Based on IP, RCU could consider those interrupted section as readers. This would > > > > > > > require PREEMPT_RCU though. > > > > > > > > > > > > > > And then synchronize_rcu() would do the 1, 2, 4 jobs. > > > > > > > > > > > > If I am following correctly (ha!), sleepable BPF programs rule out use > > > > > > of RCU in this manner. > > > > > > > > > > > > But your point is nevertheless valid, in that SRCU could be used. > > > > > > And because rcu_read_lock_trace() is a thin wrapper around SRCU-fast, we > > > > > > *might* be able to instead use rcu_read_lock_tasks_trace(), which would > > > > > > skip the task-struct increment and decrement, saving a few instructions. > > > > > > Then, instead of waiting for each task's counter to go to zero, instead > > > > > > just invoke synchronize_rcu_tasks_trace(). > > > > > > > > > > > > Which is pretty close to what Josef is proposing, just with the new RCU > > > > > > Tasks Trace read-side primitives. I think. ;-) > > > > > > > > > > > > This assumes that we do not need to flatten partially overlapping RCU > > > > > > Tasks Trace readers into one big reader. > > > > > > > > > > > > Or am I missing something here? > > > > > > > > > > Yes I think that's what Josef does in this patchset. The problem is about > > > > > handling the few instructions: > > > > > > > > > > 1) between the begining of the trampoline and the call to rcu_read_lock_trace() > > > > > > > > > > 2) between the call to rcu_read_unlock_trace() and the end of the trampoline > > > > > > > > > > So what I'm proposing is to make those two parts implicit RCU read lock sections. > > > > > > > > > > So the whole trampoline would be .text.rcu_no_qs: > > > > > > > > > > .text.rcu_no_qs trampoline: > > > > > __________________________________________________________________________________________ > > > > > |Few instructions 1 | rcu_read_lock_trace() .... rcu_read_unlock_trace | Few instructions 2| > > > > > ___________________________________________________________________________________________ > > > > > > > > > > Then when a tick fires, rcu_flavor_sched_clock_irq() discards the interrupted > > > > > code as QS if the IP was within .text.rcu_no_qs _unless_ it is in the > > > > > rcu_read_lock_trace. Both are easy and quick to verify. > > > > > > > > > > Also preempt_schedule_irq() would make sure to verify the same condition and > > > > > enqueue the task as a GP blocker if preempting inside "Few instructions 1" > > > > > or "Few instructions 2". > > > > > > > > Ah, OK, I might be following now. ;-) > > > > > > > > We also need both versions of rcu_exp_handler() to check the IP as well, > > > > given that sooner or later someone is going to want trampoline removal > > > > to go faster. Or am I still missing a turn in here somewhere? > > I should add that the thing that I really like about Frederic's approach > is that avoids the task-list scan. Or at least has the potential to > do so. Such scans have proven problematic in the past. > > > > Yes indeed, missed the exp part! > > > > What remains to handle also is non-preemptible RCU because if the task is > > preempted by an IRQ while in the .text.rcu_no_qs, we may still need to keep > > track of that somewhere. > > Perhaps in rcu_core() in kernels booted with use_softirq? I am thinking > specifically of the checks for deferred quiescent states. I don't (yet) > see a need to modify rcu_check_quiescent_state(). > > Maybe other places as well. ;-) Hmm this tracking would have to happen on preempt_schedule() just like we do for PREEMPT_RCU. Or am I missing something? And then we would need a list scan of those tasks. Or we can build the blocked task list handling, that we already have for PREEMPT_RCU, when CONFIG_RCU_TASKS && !CONFIG_PREEMPT_RCU. We would just only add tasks when preempted in .text.rcu_no_qs since rcu_read_lock() would still disable preemption on normal explicit readers. So I wouldn't expect more overhead due to that blocked list tracking built since it would rarely track tasks. Thanks. -- Frederic Weisbecker SUSE Labs