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

[Frederic Weisbecker <fweisbec@gmail.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 ?

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.