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 ABDF2C4332F for ; Sat, 10 Dec 2022 23:56:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229775AbiLJX4I (ORCPT ); Sat, 10 Dec 2022 18:56:08 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38912 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229475AbiLJX4G (ORCPT ); Sat, 10 Dec 2022 18:56:06 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C9D40CE19 for ; Sat, 10 Dec 2022 15:56:01 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 4D79C60CEB for ; Sat, 10 Dec 2022 23:56:01 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 51DCDC433D2; Sat, 10 Dec 2022 23:55:59 +0000 (UTC) Date: Sat, 10 Dec 2022 18:55:55 -0500 From: Steven Rostedt To: Thomas Gleixner Cc: "Paul E. McKenney" , linux-kernel@vger.kernel.org, Masami Hiramatsu , Andrew Morton , Karol Herbst , Pekka Paalanen , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Ingo Molnar , Borislav Petkov Subject: Re: [for-next][PATCH 13/25] x86/mm/kmmio: Use rcu_read_lock_sched_notrace() Message-ID: <20221210185555.7abc4de4@gandalf.local.home> In-Reply-To: <87359mdeg3.ffs@tglx> References: <20221210135750.425719934@goodmis.org> <20221210135825.241167123@goodmis.org> <20221210174753.GD4001@paulmck-ThinkPad-P17-Gen-1> <20221210133425.4657985e@gandalf.local.home> <87359mdeg3.ffs@tglx> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 11 Dec 2022 00:30:36 +0100 Thomas Gleixner wrote: > On Sat, Dec 10 2022 at 13:34, Steven Rostedt wrote: > > On Sat, 10 Dec 2022 09:47:53 -0800 "Paul E. McKenney" wrote: > >> This does mess with preempt_count() redundantly, but the overhead from > >> that should be way down in the noise. > > > > I was going to remove it, but then I realized that it would be a functional > > change, as from the comment above, it uses "preempt_enable_no_resched(), > > which there is not a rcu_read_unlock_sched() variant. > > preempt_enable_no_resched() in this context is simply garbage. > > preempt_enable_no_resched() tries to avoid the overhead of checking > whether rescheduling is due after decrementing preempt_count() because > the code which it this claims to know that it is _not_ the outermost one > which brings preempt count back to preemtible state. > > I concede that there are hot paths which actually can benefit, but this > code has exactly _ZERO_ benefit from that. Taking that tracing exception > and handling it is orders of magnitudes more expensive than a regular > preempt_enable(). > > So just get rid of it and don't proliferate cargo cult programming. > The point of the patch is to just fix the lockdep issue. I'm happy to remove that "no_resched" (I was planning to), but that would be a separate change, with a different purpose, and thus a separate patch. -- Steve