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 027D2C10F27 for ; Tue, 10 Mar 2020 17:26:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CAC5021927 for ; Tue, 10 Mar 2020 17:26:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1583861204; bh=bDNkvVl2IUXM6TlbwkSJ/orXHQ/JdYR8AwtLVGVWtDk=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=y6bITiC/22xqEDUU0tVmFwUQfD0zJeYThO9xO4WHXKsdLEfOy6zpLdPLEerwIVp+m y4r+a60Na7q+RqEXkVOS0Xsfl+J2qCJMQ1OfmDI+nQGdLzyrHZ5Czdq8PtcT6n9sfb kEbEFGb0Dt99n/rxZPAOzET3zraGIOae0amsEvHo= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726682AbgCJR0n (ORCPT ); Tue, 10 Mar 2020 13:26:43 -0400 Received: from mail.kernel.org ([198.145.29.99]:36006 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726271AbgCJR0n (ORCPT ); Tue, 10 Mar 2020 13:26:43 -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 9CC8E2146E; Tue, 10 Mar 2020 17:26:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1583861202; bh=bDNkvVl2IUXM6TlbwkSJ/orXHQ/JdYR8AwtLVGVWtDk=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=fWii4LBOv1kQRuFHgrE2gSUuVI3Gd95dTbma4MU3XqeS7WnRIeefoDeNSH8DVfexJ 6Alq5QRW2J0Qo5e5UQ6eJwkXwk4gCF80vLdGkbJkTxOW3Q3zFSeMGYsXmMcyisybRh IEhJ4zpJnMh8riNiyeKZ8qJ0Wi/dD2B3ptDuKr08= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 7381635229CC; Tue, 10 Mar 2020 10:26:42 -0700 (PDT) Date: Tue, 10 Mar 2020 10:26:42 -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: <20200310172642.GG2935@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> <20200310164927.GD2935@paulmck-ThinkPad-P72> <1800881789.23773.1583860965967.JavaMail.zimbra@efficios.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1800881789.23773.1583860965967.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 01:22:45PM -0400, Mathieu Desnoyers wrote: > ----- On Mar 10, 2020, at 12:49 PM, paulmck paulmck@kernel.org wrote: > > > 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? > > There is a big difference between allowing page faults to happen, and expecting > page faults to happen every time. I suspect many use-cases will end up having > a fast-path which touches user-space data which is in the page cache, but > may end up triggering page faults in rare occasions. > > Therefore, this might justify an SRCU which has low-overhead read-side. OK, good to know, thank you! Thanx, Paul