On 09/01/2007 06:07 AM, Andi Kleen wrote:
>> write_seqlock_irqsave(&vsyscall_gtod_data.lock, flags);
>> /* copy vsyscall data */
>> vsyscall_gtod_data.clock.vread = clock->vread;
>> vsyscall_gtod_data.clock.cycle_last = clock->cycle_last;
>> vsyscall_gtod_data.clock.mask = clock->mask;
>> vsyscall_gtod_data.clock.mult = clock->mult;
>> vsyscall_gtod_data.clock.shift = clock->shift;
>> vsyscall_gtod_data.wall_time_sec = wall_time->tv_sec;
>> vsyscall_gtod_data.wall_time_nsec = wall_time->tv_nsec; <===
>> vsyscall_gtod_data.sys_tz = sys_tz;
>> vsyscall_gtod_data.wall_time_nsec = wall_time->tv_nsec; <===
>
> Must have been a (harmless) merging mistake, but I bet gcc optimizes it out
> anyways.
>
I did find this after some digging:
In the vdso code:
static inline long vgetns(void)
{
cycles_t (*vread)(void);
vread = gtod->clock.vread;
return ((vread() - gtod->clock.cycle_last) * gtod->clock.mult) >>
gtod->clock.shift;
}
Looks like an open-coded version of this in the kernel timekeeping code:
static inline s64 __get_nsec_offset(void)
{
cycle_t cycle_now, cycle_delta;
s64 ns_offset;
/* read clocksource: */
cycle_now = clocksource_read(clock);
/* calculate the delta since the last update_wall_time: */
cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
/* convert to nanoseconds: */
ns_offset = cyc2ns(clock, cycle_delta);
return ns_offset;
}
But the vdso version isn't doing any masking. And the mask is different for
different clocksources, so it has to track the underlying kernel's clocksource
when it gets changed.
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