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

[Thomas Gleixner]

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.

Thanks,

	tglx