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Violators will be prosecuted for from ; Tue, 11 Aug 2015 15:50:34 -0600 Received: from b03cxnp08025.gho.boulder.ibm.com (b03cxnp08025.gho.boulder.ibm.com [9.17.130.17]) by d03dlp02.boulder.ibm.com (Postfix) with ESMTP id 7F8A13E4004E for ; Tue, 11 Aug 2015 15:50:31 -0600 (MDT) Received: from d03av04.boulder.ibm.com (d03av04.boulder.ibm.com [9.17.195.170]) by b03cxnp08025.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id t7BLnZld59113512 for ; Tue, 11 Aug 2015 14:49:35 -0700 Received: from d03av04.boulder.ibm.com (loopback [127.0.0.1]) by d03av04.boulder.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id t7BLoU61013549 for ; Tue, 11 Aug 2015 15:50:31 -0600 Date: Tue, 11 Aug 2015 14:50:29 -0700 From: "Paul E. McKenney" To: "Luis R. Rodriguez" Message-ID: <20150811215029.GI3895@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20150811184258.GB30479@wotan.suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150811184258.GB30479@wotan.suse.de> Cc: 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 Subject: Re: [Ksummit-discuss] [BELATED CORE TOPIC] context tracking / nohz / RCU state List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 ? Not sure exactly what you are suggesting, but given that many of these need to be placed in fastpaths, I am not at all excited about having to put switch statements in each of them. > 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. However, switch statements would probably be just fine for boot-time-only code. Thanx, Paul