[PATCH] D11579: [InstrProfiling] Fix data race on profile counters by using AtomicRMW.

[Sean Silva]

On Wed, Jul 29, 2015 at 6:55 PM, Bob Wilson <bob.wilson at apple.com> wrote:

>
> On Jul 29, 2015, at 4:56 PM, Alexey Samsonov <vonosmas at gmail.com> wrote:
>
> Do relaxed atomics actually introduce that much of slowdown?
>
>
> I would definitely want to see some data showing that they do not slow
> things down before we decide to do this unconditionally. We’ve discussed
> this issue several times in the past. My recollection is those discussions
> ended with an acknowledgement there is a tradeoff between speed and
> accuracy and that we don’t all agree on where we want to be on that
> spectrum. Adding options to let people choose would be one solution. Good
> data, on a variety of platforms, showing that it doesn’t make much
> difference would be another way to resolve it.
>

+1

In my testing, the overhead of the existing instrumentation is about a 2x
slowdown, which is starting to get close to the range it would be very
difficult to play an instrumented game. I wouldn't want to make this much
slower. I'm glad to test this for you; I'll try to get around to this this
week.

Also, in the past David Li suggested that his findings were that not using
atomic operations "only contribute
to very small count variations"
https://groups.google.com/d/msg/llvm-dev/ScLa2xIdo9s/Ow1FPDVVRIoJ
CC'ing David in case he has more input to the discussion.

-- Sean Silva



>
>
> You're intentionally introducing a data race, this doesn't look good to me
> at all. However, I'm not confident about
> what's allowed in LLVM IR - it's not C++ where any source-level data race
> is UB, but not an x86 assembly either.
>
> On Wed, Jul 29, 2015 at 4:43 PM, Justin Bogner <mail at justinbogner.com>
> wrote:
>
>> Alexey Samsonov <vonosmas at gmail.com> writes:
>> > samsonov created this revision.
>> > samsonov added reviewers: dnovillo, bogner.
>> > samsonov added a subscriber: llvm-commits.
>> >
>> > Since we introduced counters for functions in COMDAT sections (e.g.
>> > inline functions from STL headers), these headers can easily be
>> > incremented concurrently by multiple threads. Replace load-add-store
>> > with a single "atomicrmw add" with monotonic memory ordering.
>>
>> This significantly changes the performance characteristics of this code,
>> pessimizing single-threaded users and potentially making the
>> multithreaded performance issues even worse.
>>
>> It's fine to add an option to lower these to atomics, since this does
>> guarantee accuracy, but I think we need a switch to choose which kind of
>> lowering we're interested in in that case.
>>
>> > http://reviews.llvm.org/D11579
>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__reviews.llvm.org_D11579&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=mQ4LZ2PUj9hpadE3cDHZnIdEwhEBrbAstXeMaFoB9tg&m=lDx1rX-3B32oZTA_vCe21kRN0Y14ujW2ePnnU3JiUX4&s=tiiUXsu0al8aXyrPKSc28tcPx4YG5wxgSglC63-ASTQ&e=>
>> >
>> > Files:
>> >   lib/Transforms/Instrumentation/InstrProfiling.cpp
>> >
>> > Index: lib/Transforms/Instrumentation/InstrProfiling.cpp
>> > ===================================================================
>> > --- lib/Transforms/Instrumentation/InstrProfiling.cpp
>> > +++ lib/Transforms/Instrumentation/InstrProfiling.cpp
>> > @@ -147,9 +147,9 @@
>> >    IRBuilder<> Builder(Inc->getParent(), *Inc);
>> >    uint64_t Index = Inc->getIndex()->getZExtValue();
>> >    Value *Addr = Builder.CreateConstInBoundsGEP2_64(Counters, 0, Index);
>> > -  Value *Count = Builder.CreateLoad(Addr, "pgocount");
>> > -  Count = Builder.CreateAdd(Count, Builder.getInt64(1));
>> > -  Inc->replaceAllUsesWith(Builder.CreateStore(Count, Addr));
>> > +  Builder.CreateAtomicRMW(AtomicRMWInst::Add, Addr,
>> Builder.getInt64(1),
>> > +                          llvm::Monotonic);
>> > +  assert(Inc->use_empty() && "InstrProfIncrementInst has uses!");
>> >    Inc->eraseFromParent();
>> >  }
>>
>>
>
>
> --
> Alexey Samsonov
> vonosmas at gmail.com
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