Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751938AbbHMB3G (ORCPT ); Wed, 12 Aug 2015 21:29:06 -0400 Received: from mail-ig0-f195.google.com ([209.85.213.195]:32802 "EHLO mail-ig0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751137AbbHMB3E (ORCPT ); Wed, 12 Aug 2015 21:29:04 -0400 MIME-Version: 1.0 In-Reply-To: <20150812160342.GV3895@linux.vnet.ibm.com> References: <20150811184258.GB30479@wotan.suse.de> <20150812142732.GD21542@lerouge> <20150812160342.GV3895@linux.vnet.ibm.com> Date: Thu, 13 Aug 2015 09:29:03 +0800 Message-ID: Subject: Re: [Ksummit-discuss] [BELATED CORE TOPIC] context tracking / nohz / RCU state From: Lai Jiangshan To: Paul McKenney Cc: Frederic Weisbecker , Juergen Gross , "ksummit-discuss@lists.linuxfoundation.org" , Konrad Rzeszutek Wilk , Peter Zijlstra , "linux-kernel@vger.kernel.org" , Chris Metcalf , Jan Beulich , Christoph Lameter , Ingo Molnar 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: 3870 Lines: 87 On Thu, Aug 13, 2015 at 12:03 AM, Paul E. McKenney wrote: > On Wed, Aug 12, 2015 at 04:27:34PM +0200, Frederic Weisbecker wrote: >> On Tue, Aug 11, 2015 at 08:42:58PM +0200, Luis R. Rodriguez wrote: >> > 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 ? >> >> That's what we do with context tracking. It tracks the context (user/kernel) >> and stores these informations. And indeed the contextual informations can vary >> depending for example if an exception triggered in userspace or kernelspace. > > Another question of interest is "Can things be arranged so that RCU uses > the context-tracking information directly in place of rcu_dynticks?" > In theory, the answer is clearly "yes", but the reason that RCU's > accounting is heavyweight is the need to get precise state readout on > other CPUs. So it is quite possible that making RCU directly use the > context-tracking information will make that tracking slower and more > complex, so that the overall effect will be zero net improvement. rcu_dynticks can be directly renamed and moved to context-tracking code. ^_^. If there any other code need to access the context-tracking information, rearranging the code will be better. I once tried to use pure context-tracking information to implement rcu_sys_is_idle(), the rearranging is needed, and it is to complicated to continue. Current rcu_sys_is_idle() is complicated though. > > But it does seem worth a look. > > Thanx, Paul > > _______________________________________________ > Ksummit-discuss mailing list > Ksummit-discuss@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/ksummit-discuss -- 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/