On Fri, Oct 26, 2018 at 7:34 PM Akira Yokosawa <akiyks@xxxxxxxxx> wrote: > > On 2018/10/26 10:12:19 +0900, Akira Yokosawa wrote: > > (from mobile, might be QP encoded) > > > > 2018/10/26 7:04、Akira Yokosawa <akiyks@xxxxxxxxx> wrote: > > > >>> 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... > > > > I tried the same source with (o)+1 on another x86_64 host (Ubuntu 16.04). > > > > count_lim_atomic hog test runs flawlessly. > > > > The host I tried earlier was a bit old SB laptop. I’ll try there again later. > > And I _did_ make a stupid typo there. I tried with "(o+1)", not "(o)+1". > Apologies... > > So all clear for Junchang's patch with Paul's minor tweak. > > FWIW, diff of my version looks like the following: > > diff --git a/CodeSamples/api-pthreads/api-gcc.h b/CodeSamples/api-pthreads/api-gcc.h > index 1dd26ca..2e65998 100644 > --- a/CodeSamples/api-pthreads/api-gcc.h > +++ b/CodeSamples/api-pthreads/api-gcc.h > @@ -168,9 +168,8 @@ struct __xchg_dummy { > ({ \ > typeof(*ptr) _____actual = (o); \ > \ > - (void)__atomic_compare_exchange_n(ptr, (void *)&_____actual, (n), 1, \ > - __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST); \ > - _____actual; \ > + __atomic_compare_exchange_n(ptr, (void *)&_____actual, (n), 1, \ > + __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST) ? (o) : (o)+1; \ > }) > > static __inline__ int atomic_cmpxchg(atomic_t *v, int old, int new) > --- Hi Akira, Thanks a lot. Yes, the patch works for my x86 laptop and servers. I have submitted the path in an independent thread. Thanks, --Junchang > > Thanks, Akira > > > > > Thanks, Akira > > > >> > >> 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) > >>>>>>>> > >>>>>>> > >>>>>> > >>>>> > >>> > >> >
