Re: [Ksummit-discuss] [BELATED CORE TOPIC] context tracking / nohz / RCU state

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: "Theodore Ts'o" <tytso@mit.edu>,
	"Andy Lutomirski" <luto@amacapital.net>,
	"ksummit-discuss@lists.linuxfoundation.org"
	<ksummit-discuss@lists.linuxfoundation.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Christoph Lameter" <cl@linux.com>,
	"Frédéric Weisbecker" <fweisbec@gmail.com>,
	"Ingo Molnar" <mingo@kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Chris Metcalf" <cmetcalf@ezchip.com>,
	"Rik van Riel" <riel@redhat.com>
Subject: Re: [Ksummit-discuss] [BELATED CORE TOPIC] context tracking / nohz / RCU state
Date: Mon, 12 Oct 2015 13:40:05 -0700	[thread overview]
Message-ID: <20151012204005.GI3910@linux.vnet.ibm.com> (raw)
In-Reply-To: <20151012184051.GB29127@thunk.org>

On Mon, Oct 12, 2015 at 02:40:51PM -0400, Theodore Ts'o wrote:
> This thread had fairly intense discussion for two days, but then went dead.
> 
> Do folks think this is worth discussing at the kernel summit?

I am very interested in discussing this.  Of course, part of that
interest is due to the direct involvement of RCU code.

							Thanx, Paul

> 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
> 