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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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