From: john stultz on
On Thu, 2010-08-05 at 15:29 -0700, john stultz wrote:
> On Fri, 2010-08-06 at 00:17 +0200, Kay Sievers wrote:
> > On Thu, Aug 5, 2010 at 23:11, john stultz <johnstul(a)us.ibm.com> wrote:
> > > On Thu, 2010-08-05 at 15:33 +0300, Alexander Shishkin wrote:
> > >> On 4 August 2010 18:58, john stultz <johnstul(a)us.ibm.com> wrote:
> > >> > Is there a actual use case that you need this for? I don't really have
> > >> > an issue with the code I just really want to make sure the feature would
> > >> > be useful enough to justify the API and code maintenance going forward.
> >
> > Basically everything that schedules an action based on an absolute
> > time specification, like at 3pm today, and not in 3 hours from now,
> > needs to track such system time changes. Otherwise it has to do
> > nonsense like cron does, to wake up every minute to check the current
> > time.
>
> time_create(CLOCK_REALTIME,...) creates absolute (not relative) timers

Sorry, *timer_create*.

thanks
-john


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From: H. Peter Anvin on
On 08/05/2010 03:22 PM, Kay Sievers wrote:
>
> I think that's really awkward interface, to pass file descriptor
> numbers around and write them to magic sysfs files.
>
> I would very much prefer a file that contains the current time, and
> wakes up possible users with a POLL_ERR on changes caused by some
> other process. That works very well for things like /proc/mounts, is
> easy to get, and does not need a full page of weird instructions to
> get stuff done. :)
>

Okay, what's wrong with having a file descriptor that gets *written to*
on a notification? Why POLL_ERR?

-hpa
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From: Greg KH on
On Fri, Aug 06, 2010 at 12:22:27AM +0200, Kay Sievers wrote:
> On Thu, Aug 5, 2010 at 23:38, Greg KH <gregkh(a)suse.de> wrote:
> > On Thu, Aug 05, 2010 at 02:11:05PM -0700, john stultz wrote:
> >> On Thu, 2010-08-05 at 15:33 +0300, Alexander Shishkin wrote:
> >> > On 4 August 2010 18:58, john stultz <johnstul(a)us.ibm.com> wrote:
> >> > > Is there a actual use case that you need this for? �I don't really have
> >> > > an issue with the code I just really want to make sure the feature would
> >> > > be useful enough to justify the API and code maintenance going forward.
> >> >
> >> > Yes. What we have here is an application which takes care of different means
> >> > of time synchronization (trusted time servers, different GSM operators, etc)
> >> > and also different kinds of time-based events/notifications (like "dentist
> >> > appointment next thursday"). When it encounters a time change that is
> >> > made by some other application, it basically wants to disable automatic
> >> > time adjustment and trigger the events/notifications which are due at this
> >> > (new) time.
> >>
> >> Ok. Something specific is always more helpful then theoretical uses.
> >>
> >> I think the filtering is still a bit controversial, so you might want to
> >> respin it without that. But otherwise I'm ok with it as long as no one
> >> else objects to any of the minor details of the interface
> >>
> >> GregKH: Does /sys/kernel/time_notify seem ok by you?
> >
> > Um, it depends, what is that file going to do? �I don't see a
> > Documentation/ABI/ entry here that describes it fully :)
>
> I think that's really awkward interface, to pass file descriptor
> numbers around and write them to magic sysfs files.

Ick, really? That's not ok for sysfs.

> I would very much prefer a file that contains the current time, and
> wakes up possible users with a POLL_ERR on changes caused by some
> other process. That works very well for things like /proc/mounts, is
> easy to get, and does not need a full page of weird instructions to
> get stuff done. :)

That sounds more reasonable.

thanks,

greg k-h
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From: H. Peter Anvin on
On 08/05/2010 03:39 PM, Kay Sievers wrote:
>>
>> Okay, what's wrong with having a file descriptor that gets *written to*
>> on a notification?
>
> Because it needs documentation, and is just not needed for such a
> simple thing, I think. Why would you want to write a fd number to a
> magic file, which can be your fd right away, even passing you the data
> on read().
>

I didn't mean that, I meant a note that you open and get a pipe/socket/*.

>> Why POLL_ERR?
>
> Because normal files can not be poll()ed, and it's not that new data
> has arrived, it just tells you to rewind and read it again. It's
> commonly used:
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=5addc5dd8836aa061f6efc4a0d9ba6323726297a

It makes sense there, I guess, as some kind of sideband notification is
highly useful. Too bad we don't have a generic mechanism on files other
than inotify... a lot of things could use it (including tail, which I
think uses inotify now.)

-hpa

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From: Kay Sievers on
On Fri, Aug 6, 2010 at 00:34, H. Peter Anvin <hpa(a)zytor.com> wrote:
> On 08/05/2010 03:22 PM, Kay Sievers wrote:
>>
>> I think that's really awkward interface, to pass file descriptor
>> numbers around and write them to magic sysfs files.
>>
>> I would very much prefer a file that contains the current time, and
>> wakes up possible users with a POLL_ERR on changes caused by some
>> other process. That works very well for things like /proc/mounts, is
>> easy to get, and does not need a full page of weird instructions to
>> get stuff done. :)
>>
>
> Okay, what's wrong with having a file descriptor that gets *written to*
> on a notification?

Because it needs documentation, and is just not needed for such a
simple thing, I think. Why would you want to write a fd number to a
magic file, which can be your fd right away, even passing you the data
on read().

> Why POLL_ERR?

Because normal files can not be poll()ed, and it's not that new data
has arrived, it just tells you to rewind and read it again. It's
commonly used:
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=5addc5dd8836aa061f6efc4a0d9ba6323726297a

Kay
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