Christian Weisgerber <[email protected]> wrote:
> On 2020-06-26, George Koehler <[email protected]> wrote:
>
> > Here's macppc again. My macppc isn't using your newest diff but does
> > now need to define TC_TB in <machine/timetc.h>.
>
> Crucial question: How confident are we that TB is in sync on
> multiprocessor machines?
A few small test programs exposed the problem on alpha for me.
This one identified the problem in a glaring fashion. The upper
word of the register is per-cpu, and it toggles back and forth
upon each nanosleep call, as we return to a different cpu. (Quite
obviously the scheduler has zero afinity)
A slightly different test program was able to identify occasions
when returning to the other cpu would return a clock value which was
earlier. A variation of test_time_skew.c, I don't have it handy.
Maybe this can be quickly convered for macppc, to test?
I don't understand yet how to fix alpha. It looks like we need a
mechanism to know what cpu# we are on, and a table for the variation.
(The upper word of the alpha clock register assists with profiling
it seems, so I don't think we can put a delta there)
#include <sys/types.h>
#include <sys/time.h>
#include <stdio.h>
main()
{
struct timespec s = {0, 0};
long times[40];
volatile unsigned long t;
int i;
for (i = 0; i < sizeof times / sizeof times[0]; i++) {
__asm volatile("rpcc %0" : "=r" (t));
times[i] = t;
nanosleep(&s, NULL);
}
for (i = 0; i < sizeof times / sizeof times[0]; i++) {
t = times[i];
printf("%lx %x\n", (t >> 32) & 0xffffffff, t & 0xffffffff);
}
}
