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 90619C433F5 for ; Thu, 29 Sep 2022 19:02:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230100AbiI2TCE (ORCPT ); Thu, 29 Sep 2022 15:02:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37330 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230036AbiI2TB6 (ORCPT ); Thu, 29 Sep 2022 15:01:58 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 462B312059C for ; Thu, 29 Sep 2022 12:01:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=8Z9l+iKfRNL4ZprThx3uP/VDdA1mXNoyK+Suc+haG4E=; b=iw/G8/+MmdIo+aBqx8qKIeRBuT LwEs9SN4D2yL7KIu8lad6Rbc7ArfHYPlLtAwWv6SJfe59EHq21BPdzZiSv7VlpXHQr8eKJ5tTnHZh h8XZgUmuMb74ZBkDpid3eqhkkIwMoS9/4191S7O9fteQ1ViloqOVx5gYuyLdlGMIMk5AYbx9MorVv Xcs6p4T2ovAQxJae5TFBadcKAp49Lm4ZLtP9SDX2N5Pg2FhYWQglZXpiP0PzqB3h4Th2Z3ZZvB9jQ vO5gisSL0ocZOF5HIzkDohCN6IeA+OxAvWW314rK5ONyGz5+aLSG0AGuFtx0akj6RNwjKGZfYwMba 8u4xI8nA==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1odyn9-00DWxl-Nq; Thu, 29 Sep 2022 19:01:51 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 2C3A5300110; Thu, 29 Sep 2022 21:01:47 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 17F752BCD47B3; Thu, 29 Sep 2022 21:01:47 +0200 (CEST) Date: Thu, 29 Sep 2022 21:01:46 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: Joel Fernandes , Frederic Weisbecker , Thomas Gleixner , linux-kernel@vger.kernel.org, Boqun Feng , "Rafael J. Wysocki" Subject: Re: RCU vs NOHZ Message-ID: References: <20220915191427.GC246308@paulmck-ThinkPad-P17-Gen-1> <20220916075817.GE246308@paulmck-ThinkPad-P17-Gen-1> <20220917142508.GF246308@paulmck-ThinkPad-P17-Gen-1> <20220929152044.GE4196@paulmck-ThinkPad-P17-Gen-1> <20220929154618.GA2864141@paulmck-ThinkPad-P17-Gen-1> <20220929164204.GO4196@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220929164204.GO4196@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 29, 2022 at 09:42:04AM -0700, Paul E. McKenney wrote: > On Thu, Sep 29, 2022 at 06:23:16PM +0200, Peter Zijlstra wrote: > > On Thu, Sep 29, 2022 at 08:46:18AM -0700, Paul E. McKenney wrote: > > > On Thu, Sep 29, 2022 at 08:20:44AM -0700, Paul E. McKenney wrote: > > > > > > > > There is a directly invoked RCU hook for any transition that enables or > > > > > > disables the tick, namely the ct_*_enter() and ct_*_exit() functions, > > > > > > that is, those functions formerly known as rcu_*_enter() and rcu_*_exit(). > > > > > > > > > > Context tracking doesn't know about NOHZ, therefore RCU can't either. > > > > > Context tracking knows about IDLE, but not all IDLE is NOHZ-IDLE. > > > > > > > > > > Specifically we have: > > > > > > > > > > ct_{idle,irq,nmi,user,kernel}_enter() > > > > > > > > > > And none of them are related to NOHZ in the slightest. So no, RCU does > > > > > not have a NOHZ callback. > > > > > > > > > > I'm still thikning you're conflating NOHZ_FULL (stopping the tick when > > > > > in userspace) and regular NOHZ (stopping the tick when idle). > > > > > > Exactly how are ct_user_enter() and ct_user_exit() completely unrelated > > > to nohz_full CPUs? > > > > That's the thing; I'm not talking about nohz_full. I'm talking about > > regular nohz. World of difference there. > > And indeed, for !nohz_full CPUs, the tick continues throughout userspace > execution. But you really did have ct_user_enter() and ct_user_exit() > on your list. > > And for idle (as opposed to nohz_full userspace execution), there is still > ct_{idle,irq,nmi}_enter(). And RCU does pay attention to these. > > So exactly what are you trying to tell me here? ;-) That RCU doens't have a nohz callback -- you were arguing it does through the ct_*_enter() things, I said none of them are related to nohz.