[LLVMdev] Finding safe thread suspension points while JIT-ing (was: Add pass run listeners to the pass manager.)

[Andrew Trick]

On May 9, 2014, at 6:33 PM, Chandler Carruth <chandlerc at google.com> wrote:

> So, I'm bringing a discussion that has thus far been on llvm-commits to llvmdev because I finally (and thanks for helping me understand this Andy) understand what is *really* going on, and I think lots of others need to be aware of this and involved to figure out the right path forward.
> 
> You can find the full review thread and more context under the subject "[PATCH][PM] Add pass run listeners to the pass manager.", but here is the important bit from Juergen's initial email:
> 
> this patch provides the necessary C/C++ APIs and infastructure to enable fine-
> grain progress report and safe suspension points after each pass in the pass
> manager. 
>  
> Clients can provide a callback function to the pass manager to call after each
> pass. This can be used in a variety of ways (progress report, dumping of IR
> between passes, safe suspension of threads, etc).
> 
> I had wrongly (perhaps because of the implementation, but still, wrongly) focused on the progress report and IR dumping use cases. It sounds (from talking to Andy offline, sorry for that confusion) like the real use case is safe suspension of threads. The idea is that we would have a callback from the pass manager into the LLVMContext which would be used to recognize safe points at which the entire LLVMContext could be suspended, and to expose these callbacks through C API to JIT users.

Good. Let’s table the discussion of how to report passes and just focus on the thread suspension API. It never occurred to me that a client using the new API for thread scheduling would not *already* be making an assumption about one thread per context. I believe anything else will break these clients regardless of the API. So I didn’t see this API as imposing a new restriction. The more explicit we can be about this, the better.

> Increasingly, I can't fathom a way to get a good design for safe suspension of JIT-ing threads using callbacks tied to when passes run today. I think it is a huge mistake to bake this into the C API at this point. If you need this functionality in the C API, with a design we can use going forward, I'd like to see a *really* careful write up of exactly what the suspension point requirements are and a design for achieving them. I think it should be completely independent from any infrastructure for reporting or dumping IR in pass managers.

Yes, there absolutely needs to be a way to expose functionality within LLVM in its current form through the C API. We can say that the API works under some explicit set of rules. If some future LLVM can be configured in a way that breaks the rules, you don’t get the callback in that case.

> I think something much simpler than this might work outside of the C API, where we can essentially change how it works when we start designing how multiple threads will actually work within an LLVMContext. Would that work? Is there a way to make progress more rapidly there?
> 
> Ultimately, this is opening a huge can of worms if we put it into the C API, as I think it is going to fundamentally impact what options we actually have for parallelizing parts of LLVM in the future. If we want to go there, we need be *incredibly* explicit about what assumptions are being made here.


Let’s be explicit then.

We will always need to be able to configure LLVM with one thread per context. Always. So it’s not like we’re adding something that could become unusable in the future. Does anyone disagree?

Incidentally, I have no idea why the callback would not work with parallel context. If you suspend a thread within a thread group, it is totally expected that the other threads will also eventually block.

Tangentially, how many other places do we assume that an LLVMContext corresponds to a thread?

-Andy

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