Messages in this thread | | | Subject | Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation. | From | Christophe Leroy <> | Date | Tue, 22 Oct 2019 11:01:45 +0200 |
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Le 21/10/2019 à 23:29, Thomas Gleixner a écrit : > 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: vdso: 750 nsec/call >> >> gettimeofday: vdso: 1533 nsec/call
Small improvement (3%) with the proposed change:
gettimeofday: vdso: 1485 nsec/call
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) > { > 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 >
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