Messages in this thread | | | Date | Fri, 07 Sep 2007 15:39:25 -0400 | From | Chuck Ebbert <> | Subject | Re: 2.6.23-rc3-mm1 - vdso and gettimeofday issues with glibc |
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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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