svn commit: r280674 - user/dchagin/lemul/sys/compat/linux

[Dmitry Chagin]

2015-03-26 10:43 GMT+03:00 Bruce Evans <brde at optusnet.com.au>:

> On Thu, 26 Mar 2015, Dmitry Chagin wrote:
>
>  Log:
>>  Linux nanosleep() and clock_nanosleep() system calls always
>>  writes the remaining time into the structure pointed to by rmtp
>>  unless rmtp is NULL. The value of *rmtp can then be used to call
>>  nanosleep() again and complete the specified pause if the previous
>>  call was interrupted.
>>
>>  Note. clock_nanosleep() with an absolute time value does not write
>>  the remaining time.
>>
>
> FreeBSD doesn't even have clock_nanosleep().  It also sleeps on a wrong
> clock id (CLOCK_MONOTONIC instead of CLOCK_REALTIME) in nanosleep().
> clock_nanosleep() exists mainly because CLOCK_REALTIME is usually the
> wrong clock to sleep on, but is the one specified for nanosleep() for
> historical reasons.
>
> It is stupid for the emulator to have clock_nanosleep() before the host
> system.  The the emulator doesn't seem to have it either.  It seems to
> just use the native nanosleep(), so sleeps on the wrong clock id for all
> except CLOCK_MONOTONIC.
>

I know, Bruce. This is (clock_nanosleep) 'yet another odd job' from netbsd.
Anyway, thanks for the explanation.
I would prefer to fix host system before emulator if someone point me
to the right direction.




>
>  Modified: user/dchagin/lemul/sys/compat/linux/linux_time.c
>> ============================================================
>> ==================
>> --- user/dchagin/lemul/sys/compat/linux/linux_time.c    Thu Mar 26
>> 06:00:42 2015        (r280673)
>> +++ user/dchagin/lemul/sys/compat/linux/linux_time.c    Thu Mar 26
>> 06:36:34 2015        (r280674)
>> ...
>> @@ -490,25 +488,19 @@ linux_nanosleep(struct thread *td, struc
>>                 return (error);
>>         }
>>         error = kern_nanosleep(td, &rqts, rmtp);
>>
>
> The emulator could fix up cases where the sleep on the wrong clock is too
> short, by calculating the error and sleeping again.  It doesn't do this,
> but if it just used the correct clock id for calculating the remaining
> time for the purpose of returning it, then it would know the time to
> sleep again (when there is no error but the remaining time is > 0).
>
>  @@ -558,24 +550,19 @@ linux_clock_nanosleep(struct thread *td,
>>                 return (error);
>>         }
>>         error = kern_nanosleep(td, &rqts, rmtp);
>>
>
> Linux apparently uses the correct clock id for nanosleep(), since this
> function only tries to support that clock id (LINUX_CLOCK_REALTIME).
> The function has already returned EINVAL for other clock ids.  But since
> FreeBSD nanosleep() is broken, the unique clock id that can be supported
> here is actually LINUX_CLOCK_MONOTONIC.
>   Strictly, not even that.  FreeBSD used to use getnanouptime() in
>   nanosleep(), so nanosleep() only gave 1/HZ resolution.  There is a
>   clock id CLOCK_MONOTIC_FAST for this, so nanosleep() strictly only
>   used that clock id, and if clock_nanosleep() were implemented then
>   it should use getnanouptime() precisely when the caller uses this
>   clock id.
>
>   Presumably Linux doesn't have LINUX_CLOCK_MONOTONIC_FAST to map to
>   the FreeBSD mistake CLOCK_MONOTONIC_FAST, so the best this function
>   can do is map to CLOCK_MONOTONIC.  Changing to this would probably
>   break more than it fixes.  Apparently, Linux software does use
>   clock_nanosleep(), but with CLOCK_REALTIME, else the error for using
>   it with CLOCK_MONOTIC would be noticed.  clock_nanosleep() is not
>   very useful without TIMER_ABSTIME since it is only a verbose spelling
>   of nanosleep() then.  It is useful with TIMER_ABSTIME, but that case
>   is not supported.  Perhaps the software uses clock_nanosleep() because
>   it wants to use TIMER_ABSTIME someday when that is supported.
>
>   Now, nanosleep() uses sbintimes so it isn't clear what clock id it
>   sleeps on.  The clock is still monotonic and not affected by leap
>   seconds or suspensions.  Its resolution seems to be much lower than
>   1/HZ even when HZ is 100.  "sleep 1" often sleeps for 65 milliseconds
>   extra on freefall, but shorter sleeps rarely sleep by more than 3
>   milliseconds extra.
>
>  -       if (error != 0) {
>> -               LIN_SDT_PROBE1(time, linux_clock_nanosleep,
>> nanosleep_error,
>> -                   error);
>> -               LIN_SDT_PROBE1(time, linux_clock_nanosleep, return,
>> error);
>> -               return (error);
>> -       }
>> -
>>         if (args->rmtp != NULL) {
>> +               /* XXX. Not for TIMER_ABSTIME */
>>
>
> TIMER_ABSTIME has already been handled (by XXXing and returning an error).
>
> TIMER_ABSTIME is only very useful with CLOCK_REALTIME.  It allows sleeping
> until a specified real time without being messed up by leap seconds and
> clock steps.  For CLOCK_MONOTONIC, it is not even clear what an absolute
> time is.  I think POSIX specifies CLOCK_MONOTONIC to increment in as
> clocks to physical seconds as it can, but in FreeBSD it stops incrementing
> during suspend.
>
> Bruce
>