Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752757AbbHKSnF (ORCPT ); Tue, 11 Aug 2015 14:43:05 -0400 Received: from mx2.suse.de ([195.135.220.15]:38753 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752505AbbHKSnC (ORCPT ); Tue, 11 Aug 2015 14:43:02 -0400 Date: Tue, 11 Aug 2015 20:42:58 +0200 From: "Luis R. Rodriguez" 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 , Juergen Gross , Jan Beulich , Konrad Rzeszutek Wilk , Chris Metcalf , Rik van Riel Subject: Re: [Ksummit-discuss] [BELATED CORE TOPIC] context tracking / nohz / RCU state Message-ID: <20150811184258.GB30479@wotan.suse.de> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3212 Lines: 69 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() Can all this information be generalized with some basic core hooks or could some of this contextual informatioin typically vary depending on the sequence we are in ? It sounds like its the later and that's the issue ? Reason I ask is I've been working on a slightly different series of arch problems lately but its gotten me wondering about the possibility over adding a shared layer of hooks that some arch init code could use to relay back information about some other contextual information (in my case yielding execution in some paravirtualized scenerios, in my case I only need this during init sequences though). My reasoning for considering this didn't seem sufficient to add yet-another-layer or boilet-plate code for arch init sequence code but if there is a slew of other meta data contextual information which we could use in arch code perhaps this might make more sense then. This of course only makes sense for your use case if things really vary depending on the sequence reaching out to check for any of the above. It would not need to be tied down to init sequences alone, the way this could work for instance could be for certain critial code to feed meta data over contextual information which needs to be vetted which we currently have sloppy, or difficult waays of retrieving. Then the onus would be for all of us to vet each critial section carefully and to identify clearly all required contextual information. Luis -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/