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 CB8A8C88E56 for ; Sat, 12 Sep 2026 22:31:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Reply-To:List-Subscribe: List-Help:List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=ce1HX5koLmAjYA7zzAHVeJdUkobP3h1/gtoI/ft1GtQ=; b=wv+YcgVIQojcfwZ5IAqiSyvhzR T0hC3gCqEEwjtGUYVv1JAkBYUolqkau/mpF09VxVCjV93Tp6w//fUOC9vjbaLj3n5Or5t+DJ/7WEF +vsOVHOZZvh6YNVagm4O4Fr91a1AwzKeLAdxCsHfWPHne+yDF1uZAQTH2Ut3/iXN9eO1EJ5dmdwH2 CZbkUiebxASnKo4Dp/zFwku7clsoxjia1uEvPSd8Zbzi7EewgBIuKpS7pGmvvh33XjS/GrxS83zFS DWOcKdiQemfy5A3tFbsuXmEq7EE1A+NCNz64lscM45VJllq2++yFz0Do6Ji00cdq8i9V5aCVdG5cM uMGWCL9w==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1x5WFt-00000001FuI-2zzA; Sat, 12 Sep 2026 22:31:29 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1x5WFs-00000001Ftt-1Il1 for linux-arm-kernel@lists.infradead.org; Sat, 12 Sep 2026 22:31:28 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 03E38429C6; Sat, 12 Sep 2026 22:31:24 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5401E1F000FF; Sat, 12 Sep 2026 22:31:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789252284; bh=ce1HX5koLmAjYA7zzAHVeJdUkobP3h1/gtoI/ft1GtQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To; b=PVts4P+ZFRMNDP8IPda+oXXNIXvO3R62sGgiWrzFfvap+u4GoPR1dLdGGy1nA0Ytv 9fvL2deDyqRn/CzAhHJjEhGwZ5OAuVc1ga/S9ipJo7/P+0z1zArvcm+jE34YnTRk2r bN80Leor8ep5neouVAB55Kn+ZdojDTppn7WbzVcdKArvfbTFSNU2IgySo2UA5sFUU8 Z6dbvvFLJjFVTcEO2IO3MHptg1MK8nbOVxr+EpfImqmCw/EkpZEVQ8cipgPpzd4nsr g14pK3zn/l64SXO2vt4ABs94DMJN1Yr+vAqyP9X4MSsPkbnbKUh3IgJ94cqk9WHRfV AqhUafm/316dA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 17205CE17BD; Sat, 12 Sep 2026 15:31:24 -0700 (PDT) Date: Sat, 12 Sep 2026 15:31:24 -0700 From: "Paul E. McKenney" To: David Laight Cc: Alexei Starovoitov , Josef Bacik , Frederic Weisbecker , 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 v2 08/15] bpf, x86: Maintain Tasks RCU trampoline nesting in the BPF trampoline Message-ID: References: <20260911-b4-rcu-tasks-preempt-qs-v2-0-eaaa61ed2da4@toxicpanda.com> <20260911-b4-rcu-tasks-preempt-qs-v2-8-eaaa61ed2da4@toxicpanda.com> <14cb8a91-497d-49f5-aa20-c6cb8b9a27fc@paulmck-laptop> <8c51a669-eb6b-455b-a829-b537da5209f8@paulmck-laptop> <20260912221400.4045198b@pumpkin> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260912221400.4045198b@pumpkin> 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: , Reply-To: paulmck@kernel.org Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Sat, Sep 12, 2026 at 10:14:00PM +0100, David Laight wrote: > On Sat, 12 Sep 2026 11:03:34 -0700 > "Paul E. McKenney" wrote: > > > In the old kernels, yes, we have current->trc_reader_nesting++. > > In the newer kernels, Tasks Trace RCU is instead implemented in terms > > of SRCU-fast, which instead increments per-CPU counters. Which among > > other thins is a bit faster and does not need to hook into the scheduler. > > Isn't that rather architecture dependant? > It is fine on x86, but on arm incrementing a per-cpu variable is > significantly expensive. Last I heard, slow ARM increments of per-CPU variables were to be a transitory phenomemon. Plus changes late last year greatly sped up the per-CPU increment operations. Plus this change removed some hundreds of lines of RCU code. Thanx, Paul