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

From: Andy Lutomirski
Date: Tue Aug 11 2015 - 17:53:23 EST

On Tue, Aug 11, 2015 at 2:47 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> 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

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