Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation.

From: Christophe Leroy
Date: Sat Oct 26 2019 - 11:55:14 EST

Le 26/10/2019 Ã 15:55, Andy Lutomirski a ÃcritÂ:
On Tue, Oct 22, 2019 at 6:56 AM Christophe Leroy
<christophe.leroy@xxxxxx> wrote:

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

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 [.] _dl_addr
0.55% test_vdso [.] 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 [.] _dl_relocate_object
0.89% test_vdso [.] do_lookup_x

I've tried 'perf annotate', but I have not found how to tell perf to use 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("", RTLD_NOW | RTLD_GLOBAL);
struct timeval tv;
struct timezone tz;
int i;


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

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