[BELATED CORE TOPIC] context tracking / nohz / RCU state

[Andy Lutomirski]

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
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