Le 26/10/2019 à 17:53, Thomas Gleixner a écrit :
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.
Original PPC asm:
clock-gettime-realtime: vdso: 928 nsec/call
My original RFC:
clock-gettime-realtime: vdso: 1570 nsec/call
With your suggested vdso_calc_delta():
clock-gettime-realtime: vdso: 1512 nsec/call
With your vdso_calc_delta() and inlined do_hres():
clock-gettime-realtime: vdso: 1302 nsec/call
Christophe
