Re: [PATCH v6 00/19] Unify vDSOs across more architectures

Vincenzo Frascino
2019-06-14T12:19:21Z

On 6/14/19 1:16 PM, Thomas Gleixner wrote:
> On Tue, 4 Jun 2019, Vincenzo Frascino wrote:
>> On 31/05/2019 09:46, Arnd Bergmann wrote:
>>> One open question I touched in my review is whether we want to
>>> have a vdso version of clock_getres() in all architectures or not.
>>> I'd prefer to leave it out because there is very little advantage to
>>> it over the system call (the results don't change at runtime and
>>> can easily be cached by libc if performance ever matters), and
>>> it takes up a small amount of memory for the implementation.
>>>
>>
>> I thought about it and I ended up with what proposed in this patchset mainly for
>> symmetry across all the architectures since in the end they use the same common
>> code.
>>
>> It seems also that there is some performance impact (i.e.):
>>
>> clock-getres-monotonic: libc(system call): 296 nsec/call
>> clock-getres-monotonic: libc(vdso): 5 nsec/call
>
> clock_getres() is usually not a hot path operation.
>
>> I agree with you though when you say that caching it in the libc is a
>> possibility to overcome the performance impact.
>>
>>> We shouldn't just need it for consistency because all callers
>>> would require implementing a fallback to the system call
>>> anyway, to deal with old kernels.
>
> libc has the fallback already. Let's aim for 1:1 replacement of the
> architecture code first and then add the extra bits in separate patches.
>

Ok, thanks Thomas, I will split the patches accordingly.

> Thanks,
>
> tglx
>

--
Regards,
Vincenzo