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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 B16F6C433F5 for ; Wed, 22 Sep 2021 20:02:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9150A611C6 for ; Wed, 22 Sep 2021 20:02:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237277AbhIVUEU (ORCPT ); Wed, 22 Sep 2021 16:04:20 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49780 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237276AbhIVUES (ORCPT ); Wed, 22 Sep 2021 16:04:18 -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 3CF07C061574; Wed, 22 Sep 2021 13:02:48 -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=xMMkEjEh6E2g/7bmfUoiGSvQRpLkGpcY6eTiqRST8LI=; b=DhzinNwGViZ2LB3qjFCetI4dTu vlMTuCI/qLfhGb8MkVX1k6VGrgud5ucv6a9uDOeEx7KTk3/RgXRNbO7tzS1XvBePUE/2c5Aw7eBQs ivBVgemAdQiLZApati8fT9X6dKMlYgz4qPWps6YtUlRinWPb5FMVGGPqXF8hKEtWbEf/0DK+idU2X hcDNwOzq3y3FTZqPbTRA4fF9hT4uUsLi95lTrMXR7L1tcF+AFhxPu65Ri0EfyH3AQKInXZ3r4T/mx UZ5xTy9JFgLcWFJzryobReCmWMwcnO2bTOCEQc7dW2DwQMHiutqBchz9EzsqpovN/PHge4FqIIMfl zHZ4WGIQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1mT8Rz-0054Hx-70; Wed, 22 Sep 2021 20:02:39 +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 (2048 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id B672C300250; Wed, 22 Sep 2021 22:02:38 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 9D9072C8201A6; Wed, 22 Sep 2021 22:02:38 +0200 (CEST) Date: Wed, 22 Sep 2021 22:02:38 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: gor@linux.ibm.com, jpoimboe@redhat.com, jikos@kernel.org, mbenes@suse.cz, pmladek@suse.com, mingo@kernel.org, linux-kernel@vger.kernel.org, joe.lawrence@redhat.com, fweisbec@gmail.com, tglx@linutronix.de, hca@linux.ibm.com, svens@linux.ibm.com, sumanthk@linux.ibm.com, live-patching@vger.kernel.org Subject: Re: [RFC][PATCH 6/7] context_tracking: Provide SMP ordering using RCU Message-ID: References: <20210922110506.703075504@infradead.org> <20210922110836.244770922@infradead.org> <20210922151721.GZ880162@paulmck-ThinkPad-P17-Gen-1> <20210922195350.GC880162@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210922195350.GC880162@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org On Wed, Sep 22, 2021 at 12:53:50PM -0700, Paul E. McKenney wrote: > I wasn't saying that the patch doesn't work. But doesn't it add an IPI? > Or was I looking at it too early this morning? Ah, no. The patch allows a user-bound NOHZ_FULL task to be transitioned remotely. Unlike today, where they'll eventually poke it with a signal to force a kernel entry, which is bad m'kay :-) The code in question skips transitioning running tasks, seeing as you can't tell what they're doing etc.. Howver, with context tracking on you're supposedly able to tell they're in userspace without disturbing them -- except you really can't today. So if you can tell that a current running task is in userspace (hence my patch) you can allow the task to transition without any further ado, userspace is a safe state vs kernel text patching.