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 38293C433F5 for ; Thu, 29 Sep 2022 16:26:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235709AbiI2Q0G (ORCPT ); Thu, 29 Sep 2022 12:26:06 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57754 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236272AbiI2QZj (ORCPT ); Thu, 29 Sep 2022 12:25:39 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7C3ED13F70F for ; Thu, 29 Sep 2022 09:23:24 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; 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=KZ3fV59CesKghMkILeqQGGCDBJNNycBOEuAGsaJgiig=; b=PUugdqj6xeGCEsg1OqJcdJ+4Hb +S9oWEo2Qc/aLAOovpMWhK8/buecIbNLCifwg1d2M4SUHjfssD3hPe+ZPt7iPBjwIq2okn4EJ5Gur /PWckTC7hvWbqW2mDUtqZm+2YusKojMPBN4vV8AXNs1e+xWge9ZQo/cR/1Xrm7QOEweZdOcVkpeTm TJtlol98s0AjWUWZia0kkNKC4rcTjfcTS0DJODyir2B65B9/WTa1WA1dgrRjaeqxIPNHwz2f1QUBP ZsKblRckGqym6icO7xH9s74Ztqw99yXpI+wGVxgHWqK1N1PuT4vMmSjNV0ndmQXD6tJn+ZcLXHjLj sfr6+jjA==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1odwJh-00GwHg-G8; Thu, 29 Sep 2022 16:23:17 +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)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 22078300462; Thu, 29 Sep 2022 18:23:17 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 0CE232BCB5D70; Thu, 29 Sep 2022 18:23:17 +0200 (CEST) Date: Thu, 29 Sep 2022 18:23:16 +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: <20220915160600.GA246308@paulmck-ThinkPad-P17-Gen-1> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220929154618.GA2864141@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. nohz_full is a gimmick that shouldn't be used outside of very specific cases. Regular nohz otoh is used by everybody always.