From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752417AbbJLSlR (ORCPT ); Mon, 12 Oct 2015 14:41:17 -0400 Received: from imap.thunk.org ([74.207.234.97]:45131 "EHLO imap.thunk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932131AbbJLSlN (ORCPT ); Mon, 12 Oct 2015 14:41:13 -0400 Date: Mon, 12 Oct 2015 14:40:51 -0400 From: "Theodore Ts'o" To: Andy Lutomirski Cc: "ksummit-discuss@lists.linuxfoundation.org" , "linux-kernel@vger.kernel.org" , Paul McKenney , Christoph Lameter , =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker , Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , Peter Zijlstra , Chris Metcalf , Rik van Riel Subject: Re: [Ksummit-discuss] [BELATED CORE TOPIC] context tracking / nohz / RCU state Message-ID: <20151012184051.GB29127@thunk.org> Mail-Followup-To: Theodore Ts'o , Andy Lutomirski , "ksummit-discuss@lists.linuxfoundation.org" , "linux-kernel@vger.kernel.org" , Paul McKenney , Christoph Lameter , =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker , Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , Peter Zijlstra , Chris Metcalf , Rik van Riel References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) X-SA-Exim-Connect-IP: X-SA-Exim-Mail-From: tytso@thunk.org X-SA-Exim-Scanned: No (on imap.thunk.org); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This thread had fairly intense discussion for two days, but then went dead. Do folks think this is worth discussing at the kernel summit? thanks, - Ted On Tue, Aug 11, 2015 at 10:49:36AM -0700, Andy Lutomirski wrote: > This is a bit late, but here goes anyway. > > Having played with the x86 context tracking hooks for awhile, I think > it would be nice if core code that needs to be aware of CPU context > (kernel, user, idle, guest, etc) could come up with single, > comprehensible, easily validated set of hooks that arch code is > supposed to call. > > Currently we have: > > - RCU hooks, which come in a wide variety to notify about IRQs, NMIs, etc. > > - Context tracking hooks. Only used by some arches. Calling these > calls the RCU hooks for you in most cases. They have weird > interactions with interrupts and they're slow. > > - vtime. Beats the heck out of me. > > - Whatever deferred things Christoph keeps reminding us about. > > Honestly, I don't fully understand what all these hooks are supposed > to do, nor do I care all that much. From my perspective, the code > code should be able to do whatever it wants and rely on appropriate > notifications from arch code. It would be great if we could come up > with something straightforward that covers everything. For example: > > user_mode_to_kernel_mode() > kernel_mode_to_user_mode() > kernel_mode_to_guest_mode() > in_a_periodic_tick() > starting_nmi() > ending_nmi() > may_i_turn_off_ticks_right_now() > or, better yet: > i_am_turning_off_ticks_right_now_and_register_your_own_darned_hrtimer_if_thats_a_problem() > > Some arches may need: > > i_am_lame_and_forgot_my_previous_context() > > x86 will soon (4.3 or 4.4, depending on how my syscall cleanup goes) > no longer need that. > > Paul says that some arches need something that goes straight from IRQ > to user mode (?) -- sigh. > > etc. > > It might make sense to get enough people who understand what's going > on behind the scenes together to hash out the requirements. > > --Andy > _______________________________________________ > Ksummit-discuss mailing list > Ksummit-discuss@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/ksummit-discuss