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

[Lai Jiangshan <jiangshanlai@gmail.com>]

On Thu, Aug 13, 2015 at 12:03 AM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Wed, Aug 12, 2015 at 04:27:34PM +0200, Frederic Weisbecker wrote:
>> 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.
>
> Another question of interest is "Can things be arranged so that RCU uses
> the context-tracking information directly in place of rcu_dynticks?"
> In theory, the answer is clearly "yes", but the reason that RCU's
> accounting is heavyweight is the need to get precise state readout on
> other CPUs.  So it is quite possible that making RCU directly use the
> context-tracking information will make that tracking slower and more
> complex, so that the overall effect will be zero net improvement.

rcu_dynticks can be directly renamed and moved to context-tracking code.
^_^.

If there any other code need to access the context-tracking information,
rearranging the code will be better.

I once tried to use pure context-tracking information to
implement rcu_sys_is_idle(), the rearranging is needed,
and it is to complicated to continue. Current rcu_sys_is_idle()
is complicated though.

>
> But it does seem worth a look.
>
>                                                         Thanx, Paul
>
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