Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752342AbbHKRuA (ORCPT ); Tue, 11 Aug 2015 13:50:00 -0400 Received: from mail-oi0-f51.google.com ([209.85.218.51]:36427 "EHLO mail-oi0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751651AbbHKRt6 (ORCPT ); Tue, 11 Aug 2015 13:49:58 -0400 MIME-Version: 1.0 From: Andy Lutomirski Date: Tue, 11 Aug 2015 10:49:36 -0700 Message-ID: Subject: [BELATED CORE TOPIC] context tracking / nohz / RCU state To: "ksummit-discuss@lists.linuxfoundation.org" , "linux-kernel@vger.kernel.org" , Paul McKenney , Christoph Lameter , =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= , Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , Peter Zijlstra , Chris Metcalf , Rik van Riel Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1959 Lines: 57 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 -- 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/