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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 11233C61D85 for ; Tue, 21 Nov 2023 14:06:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234010AbjKUOGF (ORCPT ); Tue, 21 Nov 2023 09:06:05 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40660 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233976AbjKUOGB (ORCPT ); Tue, 21 Nov 2023 09:06:01 -0500 Received: from smtpout.efficios.com (smtpout.efficios.com [167.114.26.122]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5A0D2D79; Tue, 21 Nov 2023 06:05:57 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=efficios.com; s=smtpout1; t=1700575556; bh=Rjo+GzhBLOQg08qfzJaOOas6LcnZO+kRHO1x7xhCg5A=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=thxzknEYOsX54Uxq75EVE8gsqAuZckze9eVbdjGrqWhnAxFJd2zTUYbp3rlwsvm8L RvifiE+dNS7ZSLgO1vz/2iuBA+vxTg+gon+T9GxUfPeiqu1Cn1B3IDL+Ia+I3b2tn4 DUC/TT7rpYPd1WMR/O9eiC1zLaBkDDg/AWBX5uFU9AIA2HzYKaNAjYt0BqP2t5Mj4i ACdF1uP7Z+1r36tCdmelBH4eidFpnqjDGiDmFd1pe/d61iNtSmMAHFrsJwhHlsYXim cg2WHeKtjYA2VqeCjjb3bN4HawXXVIVV8MT8V9NjvagkHshjx+GeLlz0QRrCqOkWhL ZBJqug52ltvkw== Received: from [172.16.0.134] (192-222-143-198.qc.cable.ebox.net [192.222.143.198]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4SZR442Pzhz1dBP; Tue, 21 Nov 2023 09:05:56 -0500 (EST) Message-ID: Date: Tue, 21 Nov 2023 09:06:18 -0500 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 1/5] tracing: Introduce faultable tracepoints Content-Language: en-US To: Peter Zijlstra , "Paul E. McKenney" Cc: Steven Rostedt , Masami Hiramatsu , linux-kernel@vger.kernel.org, Michael Jeanson , Alexei Starovoitov , Yonghong Song , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , bpf@vger.kernel.org, Joel Fernandes References: <20231120205418.334172-1-mathieu.desnoyers@efficios.com> <20231120205418.334172-2-mathieu.desnoyers@efficios.com> <20231120214742.GC8262@noisy.programming.kicks-ass.net> <62c6e37c-88cc-43f7-ac3f-1c14059277cc@paulmck-laptop> <20231120222311.GE8262@noisy.programming.kicks-ass.net> <20231121084706.GF8262@noisy.programming.kicks-ass.net> From: Mathieu Desnoyers In-Reply-To: <20231121084706.GF8262@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023-11-21 03:47, Peter Zijlstra wrote: > On Mon, Nov 20, 2023 at 03:56:30PM -0800, Paul E. McKenney wrote: >> On Mon, Nov 20, 2023 at 11:23:11PM +0100, Peter Zijlstra wrote: >>> On Mon, Nov 20, 2023 at 02:18:29PM -0800, Paul E. McKenney wrote: >>>> On Mon, Nov 20, 2023 at 10:47:42PM +0100, Peter Zijlstra wrote: >>>>> On Mon, Nov 20, 2023 at 03:54:14PM -0500, Mathieu Desnoyers wrote: >>>>>> When invoked from system call enter/exit instrumentation, accessing >>>>>> user-space data is a common use-case for tracers. However, tracepoints >>>>>> currently disable preemption around iteration on the registered >>>>>> tracepoint probes and invocation of the probe callbacks, which prevents >>>>>> tracers from handling page faults. >>>>>> >>>>>> Extend the tracepoint and trace event APIs to allow defining a faultable >>>>>> tracepoint which invokes its callback with preemption enabled. >>>>>> >>>>>> Also extend the tracepoint API to allow tracers to request specific >>>>>> probes to be connected to those faultable tracepoints. When the >>>>>> TRACEPOINT_MAY_FAULT flag is provided on registration, the probe >>>>>> callback will be called with preemption enabled, and is allowed to take >>>>>> page faults. Faultable probes can only be registered on faultable >>>>>> tracepoints and non-faultable probes on non-faultable tracepoints. >>>>>> >>>>>> The tasks trace rcu mechanism is used to synchronize read-side >>>>>> marshalling of the registered probes with respect to faultable probes >>>>>> unregistration and teardown. >>>>> >>>>> What is trace-trace rcu and why is it needed here? What's wrong with >>>>> SRCU ? >>>> >>>> Tasks Trace RCU avoids SRCU's full barriers and the array accesses in the >>>> read-side primitives. This can be important when tracing low-overhead >>>> components of fast paths. >>> >>> So why wasn't SRCU improved? That is, the above doesn't much explain. >>> >>> What is the trade-off made to justify adding yet another RCU flavour? >> >> We didn't think you would be all that happy about having each and >> every context switch iterating through many tens or even hundreds of >> srcu_struct structures. For that matter, we didn't think that anyone >> else would be all that happy either. Us included. > > So again, what is task-trace RCU ? How does it differ from say > preemptible rcu, which AFAICT could be used here too, no? Task trace RCU fits a niche that has the following set of requirements/tradeoffs: - Allow page faults within RCU read-side (like SRCU), - Has a low-overhead read lock-unlock (without the memory barrier overhead of SRCU), - The tradeoff: Has a rather slow synchronize_rcu(), but tracers should not care about that. Hence, this is not meant to be a generic replacement for SRCU. Based on my reading of https://lwn.net/Articles/253651/ , preemptible RCU is not a good fit for the following reasons: - It disallows blocking within a RCU read-side on non-CONFIG_PREEMPT kernels, - AFAIU the mmap_sem used within the page fault handler does not have priority inheritance. Please let me know if I'm missing something. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com