Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation.

[Christophe Leroy]

Le 22/10/2019 Ã 11:01, Christophe Leroy a ÃcritÂ:
> Le 21/10/2019 Ã 23:29, Thomas Gleixner a ÃcritÂ:
> > On Mon, 21 Oct 2019, Christophe Leroy wrote:
> >
> > > This is a tentative to switch powerpc/32 vdso to generic C 
> > > implementation.
> > > It will likely not work on 64 bits or even build properly at the moment.
> > >
> > > powerpc is a bit special for VDSO as well as system calls in the
> > > way that it requires setting CR SO bit which cannot be done in C.
> > > Therefore, entry/exit and fallback needs to be performed in ASM.
> > >
> > > To allow that, C fallbacks just return -1 and the ASM entry point
> > > performs the system call when the C function returns -1.
> > >
> > > The performance is rather disappoiting. That's most likely all
> > > calculation in the C implementation are based on 64 bits math and
> > > converted to 32 bits at the very end. I guess C implementation should
> > > use 32 bits math like the assembly VDSO does as of today.
> >
> > > gettimeofday:ÂÂÂ vdso: 750 nsec/call
> > >
> > > gettimeofday:ÂÂÂ vdso: 1533 nsec/call
>
> Small improvement (3%) with the proposed change:
>
> gettimeofday:ÂÂÂ vdso: 1485 nsec/call

By inlining do_hres() I get the following:

gettimeofday:    vdso: 1072 nsec/call

Christophe

> Though still some way to go.
>
> Christophe
>
> > The only real 64bit math which can matter is the 64bit * 32bit multiply,
> > i.e.
> >
> > static __always_inline
> > u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
> > {
> > ÂÂÂÂÂÂÂÂ return ((cycles - last) & mask) * mult;
> > }
> >
> > Everything else is trivial add/sub/shift, which should be roughly the 
> > same
> > in ASM.
> >
> > Can you try to replace that with:
> >
> > static __always_inline
> > u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
> > {
> > ÂÂÂÂÂÂÂÂ u64 ret, delta = ((cycles - last) & mask);
> > ÂÂÂÂÂÂÂÂ u32 dh, dl;
> >
> > ÂÂÂÂÂÂÂÂ dl = delta;
> > ÂÂÂÂÂÂÂÂ dh = delta >> 32;
> >
> > ÂÂÂÂÂÂÂÂ res = mul_u32_u32(al, mul);
> > ÂÂÂÂÂÂÂÂ if (ah)
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ res += mul_u32_u32(ah, mul) << 32;
> >
> > ÂÂÂÂÂÂÂÂ return res;
> > }
> >
> > That's pretty much what __do_get_tspec does in ASM.
> >
> > Thanks,
> >
> > ÂÂÂÂtglx