kconfig: improve seed in randconfig

'make randconfig' uses glibc's rand function, and the seed of
that PRNG is set via:

			srand(time(NULL));

But 'time()' only increases once every second - freezing the
randconfig result within a single second.

My Nehalem testbox does randconfig much faster than 1 second
 and i have a few scripts that do 'randconfig until condition X'
loops.

Those scripts currently waste a lot of CPU time due to randconfig
changing its seed only once per second currently.

Change the seed to be micrseconds based. (I checked the statistical
spread of the seed - the now.tv_sec*now.tv_usec multiplication
there further improves it.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Roman Zippel <zippel@linux-m68k.org>
[sam: fix for systems where usec is zero - noticed by Geert Uytterhoeven]
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

1 files changed, 15 insertions, 1 deletions

diff --git a/scripts/kconfig/conf.c b/scripts/kconfig/conf.c
index 3e1057f885c6..d190092c3b6e 100644
--- a/scripts/kconfig/conf.c
+++ b/scripts/kconfig/conf.c
@@ -11,6 +11,7 @@
 #include <time.h>
 #include <unistd.h>
 #include <sys/stat.h>
+#include <sys/time.h>
 
 #define LKC_DIRECT_LINK
 #include "lkc.h"
@@ -464,9 +465,22 @@ int main(int ac, char **av)
                         input_mode = set_yes;
                         break;
                 case 'r':
+                {
+                        struct timeval now;
+                        unsigned int seed;
+
+                        /*
+                         * Use microseconds derived seed,
+                         * compensate for systems where it may be zero
+                         */
+                        gettimeofday(&now, NULL);
+
+                        seed = (unsigned int)((now.tv_sec + 1) * (now.tv_usec + 1));
+                        srand(seed);
+
                         input_mode = set_random;
-                        srand(time(NULL));
                         break;
+                }
                 case 'h':
                         printf(_("See README for usage info\n"));
                         exit(0);