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

[Andy Lutomirski]

On Tue, Aug 11, 2015 at 5:51 PM, Paul E. McKenney
<paulmck at linux.vnet.ibm.com> wrote:
> On Tue, Aug 11, 2015 at 02:52:59PM -0700, Andy Lutomirski wrote:
>> On Tue, Aug 11, 2015 at 2:47 PM, Paul E. McKenney
>> <paulmck at linux.vnet.ibm.com> wrote:
>> > On Tue, Aug 11, 2015 at 12:07:54PM -0700, Andy Lutomirski wrote:
>> >> On Tue, Aug 11, 2015 at 11:33 AM, Paul E. McKenney
>> >> <paulmck at linux.vnet.ibm.com> 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.
>> >> >
>> >> > Something about people yelling at me for waking up idle CPUs, thus
>> >> > degrading their battery lifetimes.  ;-)
>> >> >
>> >> >>  - 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.
>> >> >
>> >> > Combining these would be good, but there are subtleties.  For example,
>> >> > some arches don't have context tracking, but RCU still needs to correctly
>> >> > identify idle CPUs without in any way interrupting or awakening that CPU.
>> >> > It would be good to make this faster, but it does have to work.
>> >>
>> >> Could we maybe have one set of old RCU-only (no context tracking)
>> >> callbacks and a completely separate set of callbacks for arches that
>> >> support full context tracking?  The implementation of the latter would
>> >> presumably call into RCU.
>> >
>> > It should be possible for RCU to use context tracking if it is available
>> > and to have RCU maintain its own state otherwise, if that is what you
>> > are getting at.  Assuming that the decision is global and made at either
>> > build or boot time, anyway.  Having some CPUs tracking context and others
>> > not sounds like an invitation for subtle bugs.
>>
>> I think that, if this happens, the decision should be made at build
>> time, per arch, and not be configurable.  If x86_64 uses context
>> tracking, then I think x86_64 shouldn't need additional RCU callbacks,
>> assuming that context tracking is comprehensive enough for RCU's
>> purposes.
>
> If by "shouldn't need additional RCU callbacks" you mean that x86_64
> shouldn't need to call the existing rcu_user_enter() and rcu_user_exit()
> functions, I agree.  Ditto for rcu_irq_enter(), rcu_irq_exit(),
> rcu_nmi_enter(), rcu_nmi_exit(), I would guess.  But would be necessary
> to invoke rcu_idle_enter() and rcu_idle_exit(), especially for
> CONFIG_NO_HZ_FULL_SYSIDLE=y kernels.

Except that something wants vtime for idle, too, so maybe just
kernel_to_idle().  On the other hand, the idle loop is already fully
stocked with vtime stuff.

--Andy