[LLVMdev] Finding safe thread suspension points while JIT-ing (was: Add pass run listeners to the pass manager.)
Andrew Trick
atrick at apple.com
Thu May 15 11:32:23 PDT 2014
On May 15, 2014, at 9:50 AM, Philip Reames <listmail at philipreames.com> wrote:
> Given the use case (user mode scheduling), I'm not going to oppose this proposal. I would like to see a couple of things clarified documentation wise:
> - When is this interface valid? (i.e. the single thread case)
> - If a context does have multiple threads, is this called once per thread? Or once per thread group after internal coordination? (you can write this out of scope if desired)
> - If we later introduce multiple threads, and this mechanism doesn't support it, what will happen? Will the function just not be called?
> - You hint at this already, but clarifying the state of the current context at a suspend point would be helpful.
>
> Here's a possible draft that includes the above:
> The may-suspend callback function may be called by LLVM to transfer control back to the client that invoked the LLVM compilation. This can be used to yield control of the thread, or perform periodic work needed by the client. There is no guaranteed frequency at which callbacks must occur; in fact, the client is not guaranteed to ever receive this callback. It is at the sole discretion of LLVM to do so and only if it can guarantee that suspending the thread won't block any forward progress in other LLVM contexts.
>
> At a suspend point, the state of the current LLVM context is intentionally undefined. No assumptions about it can or should be made. In particular, call backs into the context are not supported until the suspend function returns control to LLVM. Other LLVM contexts are unaffected.
Great.
> Currently, LLVM assumes one thread per LLVM context. If, or when, we introduce multiple threads, this interface will not be available for contexts which opt-in to the thread pool model. We may extend this interface at a later time to support thread pools, but for the moment, that use case is explicitly unsupported.
Correct. We should avoid mentioning multi-thread contexts in the API docs. It is very misleading to describe a feature that LLVM does not support. We can instead add a statement to the docs explaining that the callback is only synchronous with respect to the calling thread and places no guarantee on the state of other threads, regardless of their context.
> p.s. Bikeshed wise, might "yield" be a better term than "suspend" here?
I personally like calling it “yield” because it is more intuitive and describes the use case. I proposed maySuspend because I wanted to be accurate. It is really the client deciding what to do with the callback. LLVM should make no assumption that it’s actually yielding.
Chandler likes “yield” too, so lets go with that unless anyone else wants to weigh in.
On the commits list, Juergen introduced our current use case, along with a couple other future use cases for this API. I’m sorry I neglected to clearly reiterate our usage, but when it comes to documenting the C API I intentionally try not to limit its potential to a narrow use case.
-Andy
> Philip
>
> On 05/13/2014 11:49 AM, Juergen Ributzka wrote:
>> Sounds good. Lets get started by nailing down the C API and semantics for this first.
>>
>> I mirrored the C API for the LLVM context diagnostic handler and used Andy’s suggested name for the callback.
>> The opaque handle was suggested by Duncan and can provide optional user specified information that is
>> provided back during the callback (i.e. barrier, etc).
>>
>> Cheers,
>> Juergen
>>
>> Core.h:
>> typedef void (*LLVMMaySuspendCallback)(LLVMContextRef, void *);
>>
>> /**
>> * Set the may-suspend callback function for this context.
>> *
>> * @see LLVMContext::setMaySuspendCallback()
>> */
>> void LLVMContextSetMaySuspendCallback(LLVMContextRef C,
>> LLVMMaySuspendCallback Callback,
>> void *OpaqueHandle);
>>
>> LLVMContext.h:
>> /// \brief Registers a may-suspend callback with the context.
>> ///
>> /// The may-suspend callback function may be called by LLVM to transfer
>> /// control back to the client that invoked the LLVM compilation. The client
>> /// is not garantueed to ever receive this callback. It is at the sole
>> /// discretion of LLVM to do so and only if it can guarantee that suspending
>> /// the thread won't block any forward progress in other LLVM contexts.
>> void setMaySuspendCallback(MaySuspendCallbackTy Callback, void *OpaqueHandle);
>>
>> /// \brief Calls the may-suspend callback (if applicable).
>> ///
>> /// This transfers control back to the client, which may suspend the current
>> /// thread. Only call this method when LLVM doesn't hold any global mutex or
>> /// cannot block the execution in another LLVM context.
>> void callMaySuspendCallback();
>>
>> On May 12, 2014, at 5:26 PM, Nick Lewycky <nlewycky at google.com> wrote:
>>
>>> Would you (or anyone) oppose a simple maySuspendContext() callback API? It would mean nothing more than the thread(s) for a given LLVM context can be suspended independent from other contexts.
>>>
>>> I think this is the right approach. So a given thread hits a safe point, it optionally calls a "suspend check" or "i an safe to suspend right now" callback if set. It doesn't stop other threads, it doesn't continue until the function returns.
>>>
>>> If you want to stop all threads then the user callback may contain a barrier and count down how many threads have stopped until it sees all of them.
>>>
>>> Nick
>>
>>
>>
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