Re: [GIT PULL] cputime: Full dynticks task/cputime accounting v7

[Kevin Hilman <khilman@linaro.org>]

Frederic Weisbecker <fweisbec@gmail.com> writes:

> 2013/2/15 Kevin Hilman <khilman@linaro.org>:
>> Frederic Weisbecker <fweisbec@gmail.com> writes:
>>
>>> Ingo,
>>>
>>> Please pull the new full dynticks cputime accounting code that
>>> can be found at:
>>>
>>> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
>>>       tags/full-dynticks-cputime-for-mingo
>>>
>>> My last concern is the dependency on CONFIG_64BIT. We rely on cputime_t
>>> being u64 for reasonable nanosec granularity implementation. And therefore
>>> we need a single instruction fetch to read kernel cpustat for atomicity
>>> requirement against concurrent incrementation, which only 64 bit archs
>>> can provide.
>>
>> Actually, moderately recent 32-bit ARMs can do atomic 64-bit load/stores
>> too.
>
> Does gcc automatically handle 64 bit store/loads in one way or does
> that require specific CPU instructions?

For atomic load/stores, it requires specific instructions that gcc will
not generate, so the atomic64 accessors are needed on ARM.

Kevin