What happens on timeslice overrun?

Udo A. Steinberg us15 at os.inf.tu-dresden.de
Wed Sep 21 18:09:06 CEST 2005

On Wed, 21 Sep 2005 17:32:40 +0200 Rene Wittmann (RW) wrote:

RW> When I check for timeslice overruns with global variables (set by preempter
RW> and read by rt thread)
RW> everything seems fine.
RW> Whereas when I check for deadline misses via global variables, it doesn't
RW> work.

From the example code you posted it doesn't look like you made the variable
that is shared between the two threads "volatile". Only when that variable is
volatile will the compiler read it from memory every time. Otherwise it is
free to cache the variable in a register and may not see the write from the
other thread.

But that's not the real problem here.

RW> My "real" code looks like this:
RW> extern char deadline_miss;
RW> l4_kernel_clock_t left;
RW> set_up_preempter_etc();
RW> do_reservation_and_set_up();
RW> deadline_miss = 0;
RW> while(1){
RW> 	if (deadline_miss == 0){
RW> 		l4_rt_next_period();
RW> 	}

The above part in pseudo-code:

1) read deadline miss variable
2) compare deadline miss variable against 0
3) if equal, call next_period and go to sleep until next period begins

If the deadline miss occurs somewhere after 2) and before 3) is done, then you
have the situation you describe:

RW> 	/* if I check here for deadline_miss != 0 it's true!! */
RW> 	dealine_miss = 0;

RW> Summary: when I check for deadline misses right before next_period-call,
RW> I have none. So I call next_period. If I check _again_ after the call,
RW> I have one!
RW> How could I prevent this? Waiting for the next period is not pretended as
RW> Udo stated correctly!

What you want is an atomic way of checking for a deadline miss and acting upon
the result of that check. But such functionality does not exist. There is 
currently no way you can prevent the above scenario where the deadline miss
occurs after the check but before (or while) you go to sleep via next_period.

So what you have to do instead is: upon detecting a deadline miss, the
preempter thread should ex_regs the thread with the deadline miss (thereby
cancelling the l4_rt_next_period/IPC operation). When you return from
next_period, you can check if you've been ex_regs'd out of it or whether
the call returned regularly at the beginning of your new period, e.g, by
checking the global variable that you already have.

RW> Maybe my kernel settings are also of note here: APIC with one shot mode

That's a good choice.

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