On Thu, Oct 25, 2018 at 10:11:18AM +0800, Junchang Wang wrote: > Hi Akira, > > Thanks for the mail. My understanding is that PPC uses LL/SC to > emulate CAS by using a tiny loop. Unfortunately, the LL/SC loop itself > could fail (due to, for example, context switches) even if *ptr equals > to old. In such a case, a CAS instruction in actually should return a > success. I think this is what the term "spurious fail" describes. Here > is a reference: > http://liblfds.org/mediawiki/index.php?title=Article:CAS_and_LL/SC_Implementation_Details_by_Processor_family First, thank you both for your work on this! And yes, my cmpxchg() code is clearly quite broken. > It seems that __atomic_compare_exchange_n() provides option "weak" for > performance. I tested these two solutions and got the following > results: > > 1 4 8 16 32 64 > my patch (ns) 35 34 37 73 142 281 > strong (ns) 39 39 41 79 158 313 So strong is a bit slower, correct? > I tested the performance of count_lim_atomic by varying the number of > updaters (./count_lim_atomic N uperf) on a 8-core PPC server. The > first row in the table is the result when my patch is used, and the > second row is the result when the 4th argument of the function is set > to false(0). It seems performance improves slightly if option "weak" > is used. However, there is no performance boost as we expected. So > your solution sounds good if safety is one major concern because > option "weak" seems risky to me :-) > > Another interesting observation is that the performance of LL/SC-based > CAS instruction deteriorates dramatically when the number of working > threads exceeds the number of CPU cores. If weak is faster, would it make sense to return (~o), that is, the bitwise complement of the expected arguement, when the weak __atomic_compare_exchange_n() fails? This would get the improved performance (if I understand your results above) while correctly handling the strange (but possible) case where o==n. Does that make sense, or am I missing something? Thanx, Paul > Thanks, > --Junchang > > > On Thu, Oct 25, 2018 at 6:05 AM Akira Yokosawa <akiyks@xxxxxxxxx> wrote: > > > > On 2018/10/24 23:53:29 +0800, Junchang Wang wrote: > > > Hi Akira and Paul, > > > > > > I checked the code today and the implementation of cmpxchg() looks > > > very suspicious to me; Current cmpxchg() first executes function > > > __atomic_compare_exchange_n, and then checks whether the value stored > > > in field __actual (old) has been changed to decide if the CAS > > > instruction has been successfully performed. However, I saw the *weak* > > > field is set, which, as far as I know, means > > > __atomic_compare_exchange_n could fail even if the value of *ptr is > > > equal to __actual (old). Unfortunately, current cmpxchg will treat > > > this case as a success because the value of __actual(old) does not > > > change. > > > > Thanks for looking into this! > > > > I also suspected the use of "weak" semantics of > > __atomic_compare_exchange_n(), but did not quite understand what > > "spurious fail" actually means. Your theory sounds plausible to me. > > > > I've suggested in a private email to Paul to modify the 4th argument > > to false(0) as a workaround, which would prevent such "spurious fail". > > > > Both approaches looks good to me. I'd defer to Paul on the choice. > > > > Thanks, Akira > > > > > > > > This bug happens in both Power8 and ARMv8. It seems it affects > > > architectures that use LL/SC to emulate CAS. Following patch helps > > > solve this issue on my testbeds. Please take a look. Any thoughts? > > > > > > --- > > > CodeSamples/api-pthreads/api-gcc.h | 8 +++----- > > > 1 file changed, 3 insertions(+), 5 deletions(-) > > > > > > diff --git a/CodeSamples/api-pthreads/api-gcc.h > > > b/CodeSamples/api-pthreads/api-gcc.h > > > index 1dd26ca..38a16c0 100644 > > > --- a/CodeSamples/api-pthreads/api-gcc.h > > > +++ b/CodeSamples/api-pthreads/api-gcc.h > > > @@ -166,11 +166,9 @@ struct __xchg_dummy { > > > > > > #define cmpxchg(ptr, o, n) \ > > > ({ \ > > > - typeof(*ptr) _____actual = (o); \ > > > - \ > > > - (void)__atomic_compare_exchange_n(ptr, (void *)&_____actual, (n), 1, \ > > > - __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST); \ > > > - _____actual; \ > > > + typeof(*ptr) old = (o); \ > > > + (__atomic_compare_exchange_n(ptr, (void *)&old, (n), 1, > > > __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST))? \ > > > + (o) : (n); \ > > > }) > > > > > > static __inline__ int atomic_cmpxchg(atomic_t *v, int old, int new) > > > > > >
