Messages in this thread | | | Subject | Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation. | From | Christophe Leroy <> | Date | Tue, 22 Oct 2019 15:56:33 +0200 |
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Le 22/10/2019 à 11:01, Christophe Leroy a écrit : > > > 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
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) >> { >> 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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