Re: [PATCH 1/2] vdso: sparc: stub out custom vdso implementation

[John Paul Adrian Glaubitz]

Hi,

On Mon, 2025-07-07 at 17:45 +0200, Arnd Bergmann wrote:
> On Mon, Jul 7, 2025, at 17:22, John Paul Adrian Glaubitz wrote:
> > Hello Arnd,
> > 
> > On Mon, 2025-07-07 at 16:46 +0200, Arnd Bergmann wrote:
> > > Rip out the whole thing and replace it with a minimal stub as we do
> > > on parisc and uml. This introduces a small performance regression when
> > > using a libc that is aware of the vdso (glibc-2.29 or higher).
> > 
> > Can this performance hit quantified in any way?
> 
> It's trivial to test calling glibc clock_gettime() in a loop
> on a specific piece of hardware, the difference should largely
> depend on how long the timer hardware access takes compared
> to the syscall overhead.
> 
> On machines that have neither TICK nor STICK clocksource, the
> simpler version should even be minimally faster, on those that
> have one of the two, there is an added cost for entering the
> syscall on every clock_gettime() as we do on architectures without
> vdso.

OK, thanks. Since Andreas has access to a SPARC T4 as of recently, he should
be able to test this. Please allow some time for him to review and test the
changes, so we can be sure this doesn't cause any serious regressions.

> > And is there previous serious where this change was made for other archs?
> 
> See 
> https://lore.kernel.org/lkml/2078551b-c0b0-4201-b8d7-1faafa3647e6@xxxxxxxxxxxxxxxx/#t
> for the thread that led to this one.
> 
> There is also commit 5f55e098b8d0 ("parisc: Add 64-bit gettimeofday()
> and clock_gettime() vDSO functions") that adds similar stubs on
> parisc.

OK, thanks for the pointer.

Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer
`. `'   Physicist
  `-    GPG: 62FF 8A75 84E0 2956 9546  0006 7426 3B37 F5B5 F913