Messages in this thread | | | Date | Sat, 26 Oct 2019 17:53:34 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation. |
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On Tue, 22 Oct 2019, Christophe Leroy wrote: > 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
What's the effect for clock_gettime()?
gettimeofday() is suboptimal vs. the PPC ASM variant due to an extra division, but clock_gettime() should be 1:1 comparable.
Thanks,
tglx | |