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
