[PATCH 2/2] S390: Use generic spinlock code.

[Stefan Liebler]

On 02/18/2017 06:05 PM, Torvald Riegel wrote:
> On Wed, 2017-02-15 at 17:26 +0100, Stefan Liebler wrote:
>> On 02/13/2017 09:39 PM, Torvald Riegel wrote:
>>> On Wed, 2017-02-08 at 15:49 +0100, Stefan Liebler wrote:
>>>> This is an updated version of the patch, which adjusts the s390 specific
>>>> atomic-macros in the same way as in include/atomic.h.
>>>> Thus passing a volatile int pointer is fine, too.
>>>
>>> The general direction of this is okay.
>>> Some of my comments for patch 1/2 apply here as well (eg, volatile vs.
>>> atomics).
>>>
>> See answer in patch 1/2.
>>
>>> What I don't like is the choice of 1000 for
>>> SPIN_LOCK_READS_BETWEEN_CMPXCHG.  Have you run benchmarks to come up
>>> with this value, or is it a guess?  Why isn't it documented how you end
>>> up with this number?
>>> We can keep these with a choice such as this, but then we need to have a
>>> FIXME comment in the code, explaining that this is just an arbitrary
>>> choice.
>>>
>>> I would guess that just spinning forever is sufficient, and that we
>>> don't need to throw in a CAS every now and then; using randomized
>>> exponential back-off might be more important.  This is something that we
>>> would be in a better position to answer if you'd provide a
>>> microbenchmark for this choice too.
>>> At the end of 2016, I've posted a draft of a microbenchmark for rwlocks.
>>> Maybe you can use this as a start and run a few experiments?
>>  >
>> I've run own benchmarks in the same manner as your mentioned
>> microbenchmark for rwlocks below.
>> You are right, I can't recognize a real difference between
>> #define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
>> and
>> #define SPIN_LOCK_READS_BETWEEN_CMPXCHG -1
>>
>> As it does not hurt, I prefer to use a CAS every 1000 plain reads.
>> A CAS is not necessary on current CPUs but from architecture
>> perspective, it is more correct if there is such a serialization
>> instruction.
>
> What do you mean by "more correct"?  I'm not aware of an architecture
> that would not ensure that stores on one CPU will eventually become
> visible to other CPUs.
>
> Thus, as I wrote in my review of patch 1/2, I think we should just
> remove the occassional CAS (ie, just do the equivalent of the -1
> setting, always).
>
>> There is a difference between
>> #define SPIN_LOCK_READS_BETWEEN_CMPXCHG 0
>> and one of the others.
>
> Right.  We do want to spin if the lock is acquired by another thread.
> What we should do in a future patch is to experiment with the back-off
> between loads.  tile already has some code for this, which we at least
> need to integrate at some point.
>
>> The same applies to
>> #define SPIN_TRYLOCK_LOAD_AND_TEST_BEFORE_XCHG 1
>> It does not hurt if the lock is free, but there is a difference if the
>> lock is already acquired and trylock is called often.
>
> Yes.  I've replied to this point in the 1/2 patch thread.
>
>> I've saw your microbenchmark-post and added some notes.
>
> Thanks.  I'll get to them later.
>
>> I added a FIXME comment to re-evaluate the choice once we have the
>> appropriate microbenchmarks.
>>
>>> Also, I'm not quite sure whether this number is really
>>> spinlock-specific, and I would like to find a better place for these.
>>> IMO, they should be in some header that contains default tuning
>>> parameters for synchronization code, which is provided by each
>>> architecture that uses the generic spinlock; we'd have no #ifdef for the
>>> tuning parameters, so we'd catch typos in those headers.
>>>
>> See pthread_spin_parameters.h in updated patch 1/2.
>
> I suggest that we'll work towards consensus on patch 1/2 first.  I
> believe once that is done, patch 2/2 would likely just remove s390 code.
>
I've attached an updated patch which just removes the s390 code.


> Thanks for continuing the work on this.  I know we have some back and
> forth here in terms of overall direction, but I also think we're making
> progress and are continually improving the changes.
>
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