Re: [RFC 2/3] preempt_tracer: Disable IRQ while starting/stopping due to a preempt_counter change

[Daniel Bristot de Oliveira]

On 29/05/2019 20:21, Peter Zijlstra wrote:
> On Wed, May 29, 2019 at 03:51:31PM +0200, Daniel Bristot de Oliveira wrote:
>> On 29/05/2019 12:20, Peter Zijlstra wrote:
> 
>>> I'm not sure I follow, IRQs disabled fully implies !preemptible. I don't
>>> see how the model would be more pessimistic than reality if it were to
>>> use this knowledge.
>>
>> Maybe I did not expressed myself well... and the example was not good either.
>>
>> "IRQs disabled fully implies !preemptible" is a "to big" step. In modeling (or
>> mathematical reasoning?), a good practice is to break the properties into small
>> piece, and then build more complex reasoning/implications using these "small
>> properties."
>>
>> Doing "big steps" makes you prone "miss interpretations", creating ambiguity.
>> Then, -RT people are prone to be pessimist, non-RT optimistic, and so on... and
>> that is what models try to avoid.
> 
> You already construct the big model out of small generators, this is
> just one more of those little generators.

Yes, we can take that way too...

>> For instance, explaining this using words is contradictory:>
>>> Any !0 preempt_count(), which very much includes (Hard)IRQ and SoftIRQ
>>> counts, means non-preemptible.
>>
>> One might argue that, the preemption of a thread always takes place with
>> preempt_count() != 0, because __schedule() is always called with preemption
>> disabled, so the preemption takes place while in non-preemptive.
> 
> Yeah, I know about that one; you've used it in your talks. Also, you've
> modeled the schedule preempt disable as a special state. If you want we
> can actually make it a special bit in the preempt_count word too, the
> patch shouldn't be too hard, although it would make no practical
> difference.

Good to know! I am trying not to change the code or abstractions. In the
user-space version I was using the caller address to figure out if it was a call
in the scheduler or not. In the kernel version I am planing to use a
is_sched_function() like approach. But if nothing of that works, I will explore
this possibility, thanks for the suggestion.

>> - WAIT But you (daniel) wants to fake the atomicity between preempt_disable and
>> its tracepoint!
>>
>> Yes, I do, but this is a very straightforward step/assumption: the atomicity is
>> about the real-event and the tracepoint that notifies it. It is not about two
>> different events.
>>
>> That is why it is worth letting the modeling rules to clarify the behavior of
>> system, without doing non-obvious implication in the code part, so we can have a
>> model that fits better in the Linux actions/events to avoid ambiguity.
> 
> You can easily build a little shim betwen the model and the tracehooks
> that fix this up if you don't want to stick it in the model proper.
> 
> All the information is there. Heck you can even do that 3/3 thing
> internally I think.
> 

Ack, let's see what I can came up.

Thanks!

-- Daniel