From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id 7A4993EE for ; Mon, 12 Oct 2015 18:41:09 +0000 (UTC) Date: Mon, 12 Oct 2015 14:40:51 -0400 From: Theodore Ts'o To: Andy Lutomirski Message-ID: <20151012184051.GB29127@thunk.org> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Cc: "ksummit-discuss@lists.linuxfoundation.org" , Peter Zijlstra , "linux-kernel@vger.kernel.org" , Chris Metcalf , 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: , 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