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 X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AAA77C10F27 for ; Tue, 10 Mar 2020 16:49:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 804C821D56 for ; Tue, 10 Mar 2020 16:49:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1583858969; bh=aZeeJI4m4VA8ZtSKMjKdiWAG3VUJVrzCbJacfmOigbk=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=qto4O45Vc8jQk4YVKKX0utqCrDVkAy9gyrLyT9bpHq//YtMtckYFGTmdOM/fAX5+5 ZPDhpf6CaQ7hwgABXNcuvTjNFLqYI7G0nknEDNQHz4QqalTX9LU5mkwoJ+yT3r/0c+ luSfZVdWcuoXoXQIwUB3toN2e2yNAn19llXMKbVg= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726861AbgCJQt2 (ORCPT ); Tue, 10 Mar 2020 12:49:28 -0400 Received: from mail.kernel.org ([198.145.29.99]:51602 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726307AbgCJQt2 (ORCPT ); Tue, 10 Mar 2020 12:49:28 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 3C1D321927; Tue, 10 Mar 2020 16:49:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1583858967; bh=aZeeJI4m4VA8ZtSKMjKdiWAG3VUJVrzCbJacfmOigbk=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=EN8bsFrHW5CSe5hqSMYnzI6bVJxJrFU1KeDLhy7aWdoO69m+Qh9WglObnp+WfrWbe 9NegxcjuXjpEdBF0vCmlcMxg5CENvEmjq2kHND9IeSg/1eBSeP3nHHp4W1S5KmPzMQ SQIYJvibJM6bn0EGU+a1EwuBcrVaUw37T8s5jJUc= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 190C035229CC; Tue, 10 Mar 2020 09:49:27 -0700 (PDT) Date: Tue, 10 Mar 2020 09:49:27 -0700 From: "Paul E. McKenney" To: Mathieu Desnoyers Cc: Thomas Gleixner , linux-kernel , Peter Zijlstra , rostedt , Masami Hiramatsu , Alexei Starovoitov , "Joel Fernandes, Google" , Frederic Weisbecker Subject: Re: Instrumentation and RCU Message-ID: <20200310164927.GD2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <87mu8p797b.fsf@nanos.tec.linutronix.de> <20200309204710.GU2935@paulmck-ThinkPad-P72> <379743142.23419.1583853207158.JavaMail.zimbra@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <379743142.23419.1583853207158.JavaMail.zimbra@efficios.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 10, 2020 at 11:13:27AM -0400, Mathieu Desnoyers wrote: > > > ----- On Mar 9, 2020, at 4:47 PM, paulmck paulmck@kernel.org wrote: > [...] > > > > > Suppose that we had a variant of RCU that had about the same read-side > > overhead as Preempt-RCU, but which could be used from idle as well as > > from CPUs in the process of coming online or going offline? I have not > > thought through the irq/NMI/exception entry/exit cases, but I don't see > > why that would be problem. > > > > This would have explicit critical-section entry/exit code, so it would > > not be any help for trampolines. > > > > Would such a variant of RCU help? > > > > Yeah, I know. Just what the kernel doesn't need, yet another variant > > of RCU... > > Hi Paul, > > I think that before introducing yet another RCU flavor, it's important > to take a step back and look at the tracer requirements first. If those > end up being covered by currently available RCU flavors, then why add > another ? Well, we have BPF requirements as well. > I can start with a few use-cases I have in mind. Others should feel free > to pitch in: > > Tracing callsite context: > > 1) Thread context > > 1.1) Preemption enabled > > One tracepoint in this category is syscall enter/exit. We should introduce > a variant of tracepoints relying on SRCU for this use-case so we can take > page faults when fetching userspace data. Agreed, SRCU works fine for the page-fault case, as the read-side memory barriers are in the noise compared to page-fault overhead. Back in the day, there were light-weight system calls. Are all of these now converted to VDSO or similar? > 1.2) Preemption disabled > > Tree-RCU works fine. > > 1.3) IRQs disabled > > Tree-RCU works fine. > > 2) IRQ handler context > > Tree-RCU works fine. > > 3) NMI context > > Tree-RCU works fine. > > 4) cpuidle context (!rcu_is_watching()) > > - By all means, we should not have tracepoints requiring to temporarily enable > RCU in frequent code-paths. It appears that we should be able to remove the few > offenders we currently have (e.g. enter from usermode), > - For tracepoints which are infrequently called from !rcu_is_watching context, checking > whether RCU is watching and only enabling when needed should be fast enough. > > Are there other use-cases am I missing that would justify adding another flavor of RCU ? BPF programs that might sometimes sleep, but are usually lightweight. I will be double-checking this, of course. Thanx, Paul