Le 22/10/2019 à 11:01, Christophe Leroy a écrit :

Le 21/10/2019 à 23:29, Thomas Gleixner a écrit :

quoted

On Mon, 21 Oct 2019, Christophe Leroy wrote:

quoted

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.

quoted

gettimeofday:    vdso: 750 nsec/call

gettimeofday:    vdso: 1533 nsec/call

Small improvement (3%) with the proposed change:

gettimeofday:    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

quoted

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)
{
         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)
{
         u64 ret, delta = ((cycles - last) & mask);
         u32 dh, dl;

         dl = delta;
         dh = delta >> 32;

         res = mul_u32_u32(al, mul);
         if (ah)
                 res += mul_u32_u32(ah, mul) << 32;

         return res;
}

That's pretty much what __do_get_tspec does in ASM.

Thanks,

    tglx