linux-kernel - Re: [PATCHv6 0/7] system time changes notification

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Message-ID: <AANLkTi=ozec3+z4FBn3dX7H3JfSvHanS1DLdQf02fzaa@mail.gmail.com>
Date:	Fri, 12 Nov 2010 11:47:57 +0100
From:	Kay Sievers <kay.sievers@...y.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Alexander Shishkin <virtuoso@...nd.org>, Valdis.Kletnieks@...edu,
	linux-kernel@...r.kernel.org, John Stultz <johnstul@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Greg KH <gregkh@...e.de>,
	Chris Friesen <chris.friesen@...band.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCHv6 0/7] system time changes notification

On Thu, Nov 11, 2010 at 22:16, Thomas Gleixner <tglx@...utronix.de> wrote:
> B1;2401;0cOn Thu, 11 Nov 2010, Alexander Shishkin wrote:
>
>> On Thu, Nov 11, 2010 at 03:28:13PM -0500, Valdis.Kletnieks@...edu wrote:
>> > On Thu, 11 Nov 2010 21:29:55 +0200, Alexander Shishkin said:
>> >
>> > > Consider we want stuff like "wakeup every day at 3pm", the next wakeup
>> > > might be earlier than the timer we calculated last time, on system
>> > > time changes. We need to re-calculate it. This is necessary for all
>> > > repeating events.
>> > >
>> > > Say we want to wakeup at 3pm, now it's 4pm, so we schedule it in 23
>> > > hours. Now the system time changes to 2pm, and we would expect to
>> > > wakeup in one hour, but we take 25.
>> >
>> > Sorry, I tuned in late here...
>> >
>> > So the plan is that if you're not using this new interface, it will go off at
>> > the same absolute offset (23 hours after timer was set), but if you're using
>> > this interface, your timer event gets interrupted, you get woken up (say)
>> > 15 hours into your 23, and it's your job to decide if you need to set a
>> > new timer for the remaining 6, 7, 8 hours or some other number?
>>
>> Yes. This interface doesn't deal with timers, it only provides notifications.
>
> The notification itself is pointless unless your application is
> dealing with timers which need to be adjusted the one way or the
> other.
>
> That said, I'm still not convinced that this usecase justifies a new
> systemcall.
>
> 1) We can make timers wake up when a clock change happens

That would be fine too. We just need to wake up somehow and then can
find out ourselves what has happened underneath us.

> 2) Can't we use existing notification stuff like uevents or such ?

Uevents are heavy and very expensive to handle in userspace. Udev
wakes up all of these and runs stuff. They can only be used for device
discovery or some low-frequency state change notification, nothing
else. If there is the slightest chance that they might happen at a
very high frequency, they might bring an entire box down. It's really
not what they are made for.

Can't we just let poll() on timerfd return POLLPRI|POLLERR if the time
shifted to something that needs re-calculation in the requesting
process?

Kay
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