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 2CFD0C352A5 for ; Mon, 10 Feb 2020 18:07:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 051C720842 for ; Mon, 10 Feb 2020 18:07:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581358070; bh=1sM8AtptlQsckSDtBEnBFE9IPZB3fksS+0HDhGJ+dJg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=OWUqhzIiM2YF5TpWPEJOAz2kBa5xwZt/plXNVj+rszzE/kho9so3fN1PWPV2n8gHD d1S55UW/Pc5rOaGY4/hGkcdmgQK9JSzJeF+7x0X3m1aE1cMt+JEKlxwOg1OHHZihcL Si26m//5m0+wAeLcPM2BKm/jbc6ZA2trzUKFp8Iw= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727599AbgBJSHt (ORCPT ); Mon, 10 Feb 2020 13:07:49 -0500 Received: from mail.kernel.org ([198.145.29.99]:41938 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726831AbgBJSHs (ORCPT ); Mon, 10 Feb 2020 13:07:48 -0500 Received: from paulmck-ThinkPad-P72.home (unknown [62.84.152.189]) (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 9E09A2085B; Mon, 10 Feb 2020 18:07:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1581358067; bh=1sM8AtptlQsckSDtBEnBFE9IPZB3fksS+0HDhGJ+dJg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=cMOKQJdxaFxq2ApEmJf1PevgGmgXVE5jrc8R0UHdtCxG20ESA9LmVsB2LRNB90GMy VFrD6TrDPR8bH+erOL53KbkKprziQGU4GdhWULCffOCsFJrw3P9eMU3cQxMZt6rSD8 B8dtKhbJbAx8v3aAYc7gldjffniXgQC1jeJDnNoA= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id DF6A13522700; Mon, 10 Feb 2020 10:07:45 -0800 (PST) Date: Mon, 10 Feb 2020 10:07:45 -0800 From: "Paul E. McKenney" To: Steven Rostedt Cc: Peter Zijlstra , Mathieu Desnoyers , "Joel Fernandes, Google" , linux-kernel , Greg Kroah-Hartman , "Gustavo A. R. Silva" , Ingo Molnar , Richard Fontana , Thomas Gleixner , Josh Triplett , Lai Jiangshan , Arnaldo Carvalho de Melo Subject: Re: [RFC 0/3] Revert SRCU from tracepoint infrastructure Message-ID: <20200210180745.GA4947@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> <20200210120552.1a06a7aa@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200210120552.1a06a7aa@gandalf.local.home> 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 12:05:52PM -0500, Steven Rostedt wrote: > On Mon, 10 Feb 2020 10:46:16 +0100 > Peter Zijlstra wrote: > > > Furthermore, using srcu would be detrimental, because of how it has > > smp_mb() in the read side primitives. > > I didn't realize that there was a full memory barrier in the srcu read > side. Seems to me that itself is rational for reverting it. And also a > big NAK for any suggestion to have any of the function tracing to use > it as well (which comes up here and there). Yeah, that was added some years back when people were complaining about three synchronize_sched()'s worth of latency for synchronize_srcu(). I did prepare a patch about a year ago that would allow an srcu_struct to be set up to not need the read-side smp_mb() calls, but this means longer-latency grace periods (though nowhere near as long as those of synchronize_rcu_tasks()) and also that the "fast SRCU" readers cannot be used from idle or offline CPUs. But an easy change if it is useful. Thanx, Paul