linux-kernel - Re: Incorrect behavior of timer_settime() for absolute dates in the past

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Date:	Mon, 27 Nov 2006 15:32:21 +0100
From:	John <me@...vacy.net>
To:	linux-kernel@...r.kernel.org
CC:	tglx@...esys.com, mingo@...e.hu, johnstul@...ibm.com
Subject: Re: Incorrect behavior of timer_settime() for absolute dates in the
 past

John wrote:

> I'm playing with the POSIX timers API. My platform is x86 running Linux 
> 2.6.18.1 patched with the high-resolution timer subsystem.
> 
> http://www.tglx.de/hrtimers.html
> 
> I'm seeing unexpected behavior from timer_settime().
> 
> int timer_settime(timer_t timerid, int flags,
>   const struct itimerspec *value, struct itimerspec *ovalue);
> 
> timer_settime() is used to arm a timer. If the TIMER_ABSTIME flag is 
> set, then the timer should fire when the appropriate clock reaches the 
> date specified by value. If that date is in the past, the timer should 
> fire immediately.
> 
> The Open Group Base Specifications Issue 6 states:
> http://www.opengroup.org/onlinepubs/009695399/functions/timer_getoverrun.html 
> 
> 
> "If the flag TIMER_ABSTIME is set in the argument flags, timer_settime() 
> shall behave as if the time until next expiration is set to be equal to 
> the difference between the absolute time specified by the it_value 
> member of value and the current value of the clock associated with 
> timerid. That is, the timer shall expire when the clock reaches the 
> value specified by the it_value member of value. If the specified time 
> has already passed, the function shall succeed and the expiration 
> notification shall be made."
> 
> In my tests, when timer_settime() is called with an expiration date in 
> the past, the timer still takes some time to fire.
> 
> Here's a run-down of the code provided as an attachment:
> 
> I switch to a SCHED_RR scheduling policy. In other words, whenever my 
> process wants the CPU, it gets it. (No other SCHED_RR or SCHED_FIFO 
> processes on the system.) I mask the signal that will be delivered on 
> timer expiration. I then arm a timer with an expiration date in the 
> past, check whether the signal is pending, and block waiting for the 
> signal. I then print how long I've had to wait.
> 
> # ./a.out
> RESOLUTION=1 ns
> NOW=969.735545919
> SLEEPING 1 SECOND...
> NOW=970.735581398
> NOW=970.735613525
> NOW=970.735749017
> nsdiff=135492 ns i.e. 135.5 µs
> 
> Any ideas?

Is there a better forum to discuss this matter?

Regards.

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