From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 97E542BE7BE; Wed, 16 Sep 2026 14:55:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789570504; cv=none; b=dyPft6of9Z+MiVmXgYg/IoAQLLjm1svuBXV8oHMXWgiDX2ESL7tYJPq7UgLBDmvTsihj0hxK72RrISUgejWWRNx2E2Owjk8ipRA59lMja6MAKGFIZ4P9Zj0jaIA47G2kGDIP0ceg62+gpVIG2DxkWrkJ/t0VXrClpxRj0tK6Zgs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789570504; c=relaxed/simple; bh=ihqHVFSEjm19t2440vBRUzZZeIUTaR2mEXgfGeRLT8w=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=N6nuG+BDl8eGdfIbjQe0s1Y9ZlZq0BSTfOd+y+2rYmRcDqcwe74imHoO6qcs6C1j7cbSAFNxgY/1uGdACu+nkED0eGDed4hr/Zzh6UyqROhJ5klQJZPeoFsVTGeAW34sbqVBt1PfUnIhqMpBCj7Gus+ZsTrtmRgEjhwn9uGCOqA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=WzoyCkX6; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="WzoyCkX6" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A1D151F000FF; Wed, 16 Sep 2026 14:55:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789570502; bh=QV82UNlXLIGXwYDB+EGE0wWjQDs7Krio9WNaWSO5xfQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To; b=WzoyCkX6qile7VqKWaszhfBXITo1Vva2+0mj6NaQsOyenrrABW3jFSk/WQfJacLrS swxPQeQStUtlNnTb+NFoYnsOzLHKbKCy1SDiJA95sbZqsJQVCBqHLPLNYZvvlIv8wQ erkCQQ0YASVigmBa6cSO5aiMovZ7W2Gekrh9Y1mcwIOBUe0eLhD7Fa4KhblMJQBRXy a7qqgENAA49mGyM3i2AajSuIwgC2KGSJh0IxZtlr6wQmpfStpfaz9d875V82OB/h92 B0Oy45XssMrBhorahUjjMDJS+76TYEIiUGYH0KMWUCQWE0ASKp0vHT5E8Cr30QCw1+ pTumMZ0fkABgw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 22DAECE04DE; Wed, 16 Sep 2026 07:55:02 -0700 (PDT) Date: Wed, 16 Sep 2026 07:55:02 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker 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: <6afb0afe-e7b8-46b4-9fea-2c7cf4791044@paulmck-laptop> Reply-To: paulmck@kernel.org 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> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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. ;-) Thanx, Paul