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 B49D1C352A3 for ; Mon, 10 Feb 2020 13:37:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8BC772085B for ; Mon, 10 Feb 2020 13:37:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581341822; bh=nB7K6kpxye76S3Jb8ETTHL7QodfEBqCDjkWsssZduzE=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=PcN/cytndSnx3f6vUFu7971feNKxelaYhf0AxS7Xv4YKBYzVP8DM1ItDvdTfiy48z UPw3dbrm0f7XtEK8sNLHgdtaNccwTS1J8zJTYxbNi3Rp8gPubAu3gfgWu9Teq14/EK CK15oXyK6YZl0KrrorITj0vQsJ75r9trCqpjV1qA= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731706AbgBJNhB (ORCPT ); Mon, 10 Feb 2020 08:37:01 -0500 Received: from mail.kernel.org ([198.145.29.99]:35542 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727695AbgBJNg6 (ORCPT ); Mon, 10 Feb 2020 08:36:58 -0500 Received: from paulmck-ThinkPad-P72.home (unknown [193.85.242.128]) (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 5FB6120714; Mon, 10 Feb 2020 13:36:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581341817; bh=nB7K6kpxye76S3Jb8ETTHL7QodfEBqCDjkWsssZduzE=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=qL0E8isKyNkFKrWHVRu6fEZ8YKQGgpCkWyrklytg5HLFiuRVbWyMcACAdGDyWyvKL tXWa6K4S1TnaIaRebnAETuYUEj8+xmvoxHFOJ5wFKfj8iGowNk3Di/J1v98QyddiKJ k5TPJaaxpzQm/HzKDSK8Ary0+sya0jZdvGiHG7/E= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 02E833522700; Mon, 10 Feb 2020 05:36:53 -0800 (PST) Date: Mon, 10 Feb 2020 05:36:52 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Mathieu Desnoyers , "Joel Fernandes, Google" , linux-kernel , Greg Kroah-Hartman , "Gustavo A. R. Silva" , Ingo Molnar , Richard Fontana , rostedt , Thomas Gleixner , Josh Triplett , Lai Jiangshan , Arnaldo Carvalho de Melo Subject: Re: [RFC 0/3] Revert SRCU from tracepoint infrastructure Message-ID: <20200210133652.GV2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200207205656.61938-1-joel@joelfernandes.org> <1997032737.615438.1581179485507.JavaMail.zimbra@efficios.com> <20200210094616.GC14879@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200210094616.GC14879@hirez.programming.kicks-ass.net> 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 Mon, Feb 10, 2020 at 10:46:16AM +0100, Peter Zijlstra wrote: > On Sat, Feb 08, 2020 at 11:31:25AM -0500, Mathieu Desnoyers wrote: > > ----- On Feb 7, 2020, at 3:56 PM, Joel Fernandes, Google joel@joelfernandes.org wrote: > > > > > Hi, > > > These patches remove SRCU usage from tracepoints. The reason for proposing the > > > reverts is because the whole point of SRCU was to avoid having to call > > > rcu_irq_enter_irqson(). However this was added back in 865e63b04e9b2 ("tracing: > > > Add back in rcu_irq_enter/exit_irqson() for rcuidle tracepoints") because perf > > > was breaking.. > > > > I think the original patch re-enabling the rcu_irq_enter/exit_irqson() is a > > tracepoint band-aid over what should actually been fixed within perf instead. > > > > Perf should not do rcu_read_lock/unlock()/synchronize_rcu(), but rather use > > tracepoint_synchronize_unregister() to match the read-side provided by > > tracepoints. > > > > If perf can then just rely on the underlying synchronization provided by each > > instrumentation providers (tracepoint, kprobe, ...) and not explicitly add its own > > unneeded synchronization on top (e.g. rcu_read_lock/unlock), then it should simplify > > all this. > > It can't. At this point it doesn't know where the event came from. Also, > the whole perf stuff is per definition non-preemptible, as it needs to > run from NMI context. > > Furthermore, using srcu would be detrimental, because of how it has > smp_mb() in the read side primitives. Note that rcu_irq_enter() and rcu_irq_exit() also contain value-returning atomics, which imply full memory barriers. > The best we can do is move that rcu_irq_enter/exit_*() crud into the > perf tracepoint glue I suppose. One approach would be to define a synchronize_preempt_disable() that waits only for pre-existing disabled-preemption regions (including of course diabled-irq and NMI-handler regions. Something like Steve Rostedt's workqueue-baed schedule_on_each_cpu(ftrace_sync) implementation might work. There are of course some plusses and minuses: + Works on preempt-disable regions in idle-loop code without the need to invoke rcu_idle_exit() and rcu_idle_enter().. + Straightforward implementation. - Does not work on preempt-disable regions on offline CPUs. (I have no idea if this really matters.) - Schedules on idle CPUs, so usage needs to be restricted to avoid messing up energy-efficient systems. (It should be just fine to use this for tracing.) Thoughts? Thanx, Paul