"hwclock -w" takes 24 seconds

[Piotr Grudzinski]

> On Wednesday 14 April 2010 01:01:08 Steve Bennett wrote:
>> On 13/04/2010, at 11:09 AM, Rob Landley wrote:
>> > On Tuesday 13 April 2010 10:08:56 Kim B. Heino wrote:
>> >> BusyBox 1.16.1 on a small armv4tl system:
>> >>
>> >> $ time hwclock -w
>> >> real    0m 24.34s
>> >> user    0m 0.00s
>> >> sys     0m 0.00s
>> >>
>> >> $ time hwclock -w
>> >> real    0m 24.07s
>> >> user    0m 0.01s
>> >> sys     0m 0.00s
>> >>
>> >> $ time hwclock -w
>> >> real    0m 24.20s
>> >> user    0m 0.00s
>> >> sys     0m 0.00s
>> >>
>> >> rem_usec seems to be about 996600 after every iteration. Changing sync
>> >> resolution from 1ms to 5ms helps, but there has to be better solution.
>> >> Denys?
>> >
>> > I can confirm this on my armv4tl system image:
>> >
>> >  wget
>> > http://impactlinux/com/fwl/downloads/binaries/system-image-armv4tl.tar.bz
>> >2 tar xvjf system-image-armv4tl.tar.bz2
>> >  cd system-image-armv4tl
>> >  ./run-emulator.sh
>> >
>> > wait through the boot messages...
>> >
>> >  (armv4tl) /home # time hwclock -w
>> >  real        0m 24.98s
>> >  user       0m 0.01s
>> >  sys         0m 0.01s
>> >
>> > This assumes you have qemu 0.12.x installed.
>> >
>> > Rob
>> >
>> > P.S: As expected, application emulation is useless here:
>> >
>> >  $ qemu-arm ./hwclock
>> >  Unsupported ioctl: cmd=0xffffffff80247009
>> >  hwclock: RTC_RD_TIME: Function not implemented
>>
>> Interesting.
>>
>> BusyBox v1.17.0.git
>>
>> armv5teb
>> ~ # strace hwclock -w
>> ... blah blah ...
>> open("/etc/adjtime", O_RDONLY)          = -1 ENOENT (No such file or
>> directory) open("/dev/rtc", O_WRONLY)              = -1 ENOENT (No such
>> file or directory) open("/dev/rtc0", O_WRONLY)             = 3
>> gettimeofday({1271224598, 269609}, NULL) = 0
>> open("/etc/config/TZ", O_RDONLY)        = 4
>> read(4, "EST-10\n", 68)                 = 7
>> read(4, "", 61)                         = 0
>> close(4)                                = 0
>>
>> ... lots of these ...
>>
>> open("/etc/config/TZ", O_RDONLY)        = 4
>> read(4, "EST-10\n", 68)                 = 7
>> read(4, "", 61)                         = 0
>> close(4)                                = 0
>> gettimeofday({1271224622, 14271}, NULL) = 0
>> nanosleep({0, 984729000}, NULL)         = 0
>> gettimeofday({1271224623, 8166}, NULL)  = 0
>
> That's a largeish looking nanosleep.
>
> At a guess, that would be this util-linx/hwclock.c snippet:
>
>                /* Try to sync up by sleeping */
>                rem_usec = 1000000 - tv.tv_usec;
>                if (rem_usec < 1024) {
>                        goto small_rem; /* already close, don't sleep */
>                }
>                /* Need to sleep.
>                 * Note that small adj on slow processors can make us
>                 * to always overshoot tv.tv_usec < 1024 check on next
>                 * iteration. That's why adj is increased on each 
> iteration.
>                 * This also allows it to be reused as a loop limiter.
>                 */
>                usleep(rem_usec - adj);
>
>
> I'm guessing the tv.tv_usec is larger than 1000000 and thus it wraps?
>
> When CELF is over, I should look at what gettimeofday() is returning to 
> fill in
> the tv field.  Right now, I'm at a conference and thus a bit 
> overscheduled...
>
> Rob

If the condition (rem_usec < 1024) is never met, which seems to be the case 
on my system
with timer tick of 10ms, it will take (1024-200)/32=25.75 seconds to execute 
the hwclock
command.

Best Regards,
Piotr