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Re: [PATCH] powerpc: Only enable TLE with PPC_FEATURE2_HTM_NOSC

On 27/08/2018 19:44, Tulio Magno Quites Machado Filho wrote:
> Adhemerval Zanella <> writes:
>> diff --git a/sysdeps/powerpc/nptl/tls.h b/sysdeps/powerpc/nptl/tls.h
>> index f88fed5..8db970d 100644
>> --- a/sysdeps/powerpc/nptl/tls.h
>> +++ b/sysdeps/powerpc/nptl/tls.h
>> @@ -67,8 +67,7 @@ typedef struct
>>    uint32_t padding;
>>    uint32_t at_platform;
>>  #endif
>> -  /* Indicate if HTM capable (ISA 2.07).  */
>> -  uint32_t tm_capable;
>> +  uint32_t __unused1;
> Is the TCB part of the library ABI?
>> diff --git a/sysdeps/unix/sysv/linux/powerpc/elision-conf.c b/sysdeps/unix/sysv/linux/powerpc/elision-conf.c
>> index 906882a..508b917 100644
>> --- a/sysdeps/unix/sysv/linux/powerpc/elision-conf.c
>> +++ b/sysdeps/unix/sysv/linux/powerpc/elision-conf.c
>> @@ -127,6 +127,30 @@ elision_init (int argc __attribute__ ((unused)),
>>  	       TUNABLE_CALLBACK (set_elision_skip_trylock_internal_abort));
>>  #endif
>> +  /* Linux from 3.9 through 4.2 does not abort HTM transaction on syscalls,

In fact it should be 4.1, 4.2 is the first release which actually abort transactions
on syscalls.

>> +     instead it suspend and resume it when leaving the kernel.  The
>> +     side-effects of the syscall will always remain visible, even if the
>> +     transaction is aborted.  This is an issue when transaction is used along
>> +     with futex syscall, on pthread_cond_wait for instance, where the futex
>> +     call might succeed but the transaction is rolled back leading the
>> +     pthread_cond object in an inconsistent state.
>> +
>> +     GLIBC used to prevent it by always aborting a transaction before issuing
>> +     a syscall.  Linux 4.2 also decided to abort active transaction in
>> +     syscalls which makes the GLIBC workaround superflours.  Worse, GLIBC
>> +     transaction abortion leads to a performance issue on recent kernels
>> +     where the HTM state is saved/restore lazily.  By aborting a transaction
>> +     on every syscalls, regardless whether a transaction has being initiated
>> +     before, glibc make the kernel always save/restore HTM state (it can not
>> +     even lazily disable it after a certain number of syscall iterations).
>> +
>> +     Because of this shortcoming, Lock Elision is just enabled when it has
>> +     been explictly set (either by tunables of by a configure switch) and if
>> +     kernel aborts HTM transactions on syscalls (PPC_FEATURE2_HTM_NOSC)  */
>> +
>> +  __pthread_force_elision = __pthread_force_elision &&
>> +			    GLRO (dl_hwcap2) & PPC_FEATURE2_HTM_NOSC;
> In other words: this patch is requiring Linux 4.3 for TLE.
> Can't ABORT_TRANSACTION check for PPC_FEATURE2_HTM_NOSC in tcbhead_t.hwcap and
> continue to support older Linux versions?

I don't have a strong opinion here, but chatting with Breno I had the impression
that TLE (and HTM in general) is only used on kernel with this recent fix. This
patch is really a hack insertion that penalizes all syscall for an specific
feature which may not be present in the kernel. It is unfortunate that kernel
developers did not realize at feature inclusion this design for transaction 
state is not the best one for library usage.

Anyhow, another possible fix would be to set tcbhead::tm_capable to 0 in case
of PPC_FEATURE2_HTM_NOSC (and also maybe rename to something like tm_nosc).

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