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 1332144236C; Wed, 16 Sep 2026 12:40:23 +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=1789562425; cv=none; b=TL8zMZDoIxDrBXXP/7ZbjR5ijc70CnXePpNCRLvyxZeYjhQKXXVzgLE6j8kCYO/IkzR2w0Duk7BSaIoarFRpQMrxNbANhAGTMHijtjDc+t/tgJPHsAi5vDuNngnDZEm/FhNPCAtADGW+v74zC0KzinPNyOH2nnThm7xrbsmAJ00= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789562425; c=relaxed/simple; bh=a4JUtXJqVikB5UyDmPfb5Qk9gUdAc1p/iBRI/NMdXDU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Q3Fi1ghyOEnOeagHITTCfptN8MqYPNuJheMH0qrPlk1N+0z+CougUgPOcSBsRB6v/iPO7keK3/Ml8Ke8UmjjpfT7ebbv3kbuI2FUa4ots+yIqN3xpczuq0bq0rFA7m/Ea16SoUcgqZQCXbVT1umYQamMmMb9JtWxBG2t5RtUkOA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=WIJFRT40; 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="WIJFRT40" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 897C31F000FF; Wed, 16 Sep 2026 12:40:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789562423; bh=7AU/8ACJEmdfMEzWFUoLouoFn9hVgqhFaXoJr5gmXy0=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=WIJFRT40PNWCKpJEGdeyOHWQs2coLd0zbqmAGheVUz0XzbaIhFkUQ1tuVTMRTz0IE 9MKQlzG6J1v0UfcJygkPvKJozwlsQrnUjaMakzWadv4L6dGulYSUqvENefsZ2cIURx e3g6QyEiLDyESMbbYUSKrphJEieWL+uqCprVnN7n61v1eo3Qg6lnoP5k+NPMgemXUJ y9/V4fTq+wWUnUogN5WSpt3wM0wK5axcpILVbmVA1++yV815rVyxfVEneI0GYu8Ze5 ase2yrBBtCFqdkglLPiLLuC5iqEDJ6i++91EG0Qfy6A1twPB1cUhwrqml2KebAgEMk TQAOGzSqc0DtQ== Date: Wed, 16 Sep 2026 14:40:20 +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> Precedence: bulk X-Mailing-List: linux-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: 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". And since RCU tasks already does a synchronize RCU before and after the scan, that's all we would have to do. Thanks. -- Frederic Weisbecker SUSE Labs