Le 26/10/2019 à 15:55, Andy Lutomirski a écrit :

On Tue, Oct 22, 2019 at 6:56 AM Christophe Leroy
[off-list ref] wrote:

quoted

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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

A perf report might be informative.

Not sure there is much to learn from perf report:

With the original RFC:

     51.83%  test_vdso  [vdso]             [.] do_hres
     24.86%  test_vdso  [vdso]             [.] __c_kernel_gettimeofday
      7.33%  test_vdso  [vdso]             [.] __kernel_gettimeofday
      5.77%  test_vdso  test_vdso          [.] main
      1.55%  test_vdso  [kernel.kallsyms]  [k] copy_page
      0.67%  test_vdso  libc-2.23.so       [.] _dl_addr
      0.55%  test_vdso  ld-2.23.so         [.] do_lookup_x

With vdso_calc_delta() optimised as suggested by Thomas + inlined do_hres():

     68.00%  test_vdso  [vdso]             [.] __c_kernel_gettimeofday
     12.59%  test_vdso  [vdso]             [.] __kernel_gettimeofday
      6.22%  test_vdso  test_vdso          [.] main
      2.07%  test_vdso  [kernel.kallsyms]  [k] copy_page
      1.04%  test_vdso  ld-2.23.so         [.] _dl_relocate_object
      0.89%  test_vdso  ld-2.23.so         [.] do_lookup_x

I've tried 'perf annotate', but I have not found how to tell perf to use 
vdso32.so.dbg file for annotate [vdso].

Test app:

#include <dlfcn.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/time.h>

static int (*gettimeofday_vdso)(struct timeval *tv, struct timezone *tz);

int main(int argc, char **argv)
{
	void *handle = dlopen("linux-vdso32.so.1", RTLD_NOW | RTLD_GLOBAL);
	struct timeval tv;
	struct timezone tz;
	int i;

	(void)dlerror();

	gettimeofday_vdso = dlsym(handle, "__kernel_gettimeofday");
	if (dlerror())
		gettimeofday_vdso = NULL;

	for (i = 0; i < 100000; i++)
		gettimeofday_vdso(&tv, &tz);
}


Christophe