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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 08AAAC433DF for ; Tue, 19 May 2020 21:20:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D2EC52072C for ; Tue, 19 May 2020 21:20:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726658AbgESVUv (ORCPT ); Tue, 19 May 2020 17:20:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36758 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725998AbgESVUu (ORCPT ); Tue, 19 May 2020 17:20:50 -0400 Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7508CC08C5C0 for ; Tue, 19 May 2020 14:20:50 -0700 (PDT) Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1jb9et-0002Ce-VC; Tue, 19 May 2020 23:20:20 +0200 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id 15BAD100D01; Tue, 19 May 2020 23:20:19 +0200 (CEST) From: Thomas Gleixner To: Andy Lutomirski Cc: Andy Lutomirski , LKML , X86 ML , "Paul E. McKenney" , Alexandre Chartre , Frederic Weisbecker , Paolo Bonzini , Sean Christopherson , Masami Hiramatsu , Petr Mladek , Steven Rostedt , Joel Fernandes , Boris Ostrovsky , Juergen Gross , Brian Gerst , Mathieu Desnoyers , Josh Poimboeuf , Will Deacon , Tom Lendacky , Wei Liu , Michael Kelley , Jason Chen CJ , Zhao Yakui , "Peter Zijlstra \(Intel\)" Subject: Re: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond_rcu() In-Reply-To: References: <20200515234547.710474468@linutronix.de> <20200515235125.628629605@linutronix.de> <87ftbv7nsd.fsf@nanos.tec.linutronix.de> <87a7237k3x.fsf@nanos.tec.linutronix.de> Date: Tue, 19 May 2020 23:20:19 +0200 Message-ID: <874ksb7hbg.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Andy Lutomirski writes: > On Tue, May 19, 2020 at 1:20 PM Thomas Gleixner wrote: >> Thomas Gleixner writes: >> It's about this: >> >> rcu_nmi_enter() >> { >> if (!rcu_is_watching()) { >> make it watch; >> } else if (!in_nmi()) { >> do_magic_nohz_dyntick_muck(); >> } >> >> So if we do all irq/system vector entries conditional then the >> do_magic() gets never executed. After that I got lost... > > I'm also baffled by that magic, but I'm also not suggesting doing this > to *all* entries -- just the not-super-magic ones that use > idtentry_enter(). > > Paul, what is this code actually trying to do? Citing Paul from IRC: "The way things are right now, you can leave out the rcu_irq_enter() if this is not a nohz_full CPU. Or if this is a nohz_full CPU, and the tick is already enabled, in that case you could also leave out the rcu_irq_enter(). Or even if this is a nohz_full CPU and it does not have the tick enabled, if it has been in the kernel less than a few tens of milliseconds, still OK to avoid invoking rcu_irq_enter() But my guess is that it would be a lot simpler to just always call it. Hope that helps. Thanks, tglx