On 2018/10/26 0:17, Akira Yokosawa wrote: > On 2018/10/25 23:09, Junchang Wang wrote: >> On Thu, Oct 25, 2018 at 5:45 PM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> wrote: >>> >>> 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? >> >> Hi Paul and Akira, >> >> Yes, the weak version is faster. The solution looks good. But when I >> tried to use the following patch >> >> #define cmpxchg(ptr, o, n) \ >> ({ \ >> typeof(*ptr) old = (o); \ >> (__atomic_compare_exchange_n(ptr, (void *)&old, (n), 1, > > You need a "\" at the end of the line above. (If it was not unintentionally > wrapped.) > > If it was wrapped by your mailer, which is troublesome in sending patches, > please refer to: > > https://www.kernel.org/doc/html/latest/process/email-clients.html. > >> __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST))? \ >> (o) : (~o); \ >> }) >> >> gcc complains of my use of complement symbol >> >> ../api.h:769:12: error: wrong type argument to bit-complement >> (o) : (~o); \ >> ^ >> >> Any suggestions? > > I don't see such error if I add the "\" mentioned above. > Or do you use some strict error checking option of GCC? Ah, I see that the error in compiling CodeSamples/advsync/q.c. The call site is: struct el *q_pop(struct queue *q) { struct el *p; struct el *pnext; for (;;) { do { p = q->head; pnext = p->next; if (pnext == NULL) return NULL; } while (cmpxchg(&q->head, p, pnext) != p); if (p != &q->dummy) return p; q_push(&q->dummy, q); if (q->head == &q->dummy) return NULL; } } In this case, p and pnext are pointers, hence the error. returning (o)+1 instead should be OK in this case. But now, "count_lim_atomic 3 hog" says: FAIL: only reached -1829 rather than 0 on x86_64. Hmm. No such error is observed on POWER8. Hmm... The strong version works both on x86_64 and POWER8. Thanks, Akira > > Thanks, Akira > >> >> Thanks, >> --Junchang >> >> >>> >>> 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) >>>>>> >>>>> >>>> >>> >
