[LITMUS^RT] Budget Consumed By Jobs In Ready Queue

[Jeremy Erickson]

On Fri, Aug 24, 2012 at 4:24 AM, Björn Brandenburg <bbb at mpi-sws.org> wrote:

>
> No, GSN-EDF implements link-based scheduling to support non-preemptive
> sections. A job is in the ready queue if it is not _linked_. A linked job
> may still be scheduled, either while it is non-preemptive or when the
> actual system is still catching up to the ideal system (e.g., if the
> rescheduling IPI is still in flight).
>
> Thus, queued jobs may consume budget since they could still be scheduled
> (for some short time).
>
> - Björn
>

So what's been happening to me is that I've been getting unlucky and having
jobs get unlinked right before they run out of budget.  Apparently
something about my scheduler and/or tests makes that more likely than it
usually is, but it should have a nonzero probability even with the normal
scheduler and tests.  When that happens, it triggers the BUG_ON in
arm_enforcement_timer in budget.c as soon as the (now-exhausted) job is
taken off the ready queue and scheduled.

I think this is a minor bug that should be fixed in mainline LITMUS^RT.  I
have attached two possible fixes: the first triggers a reschedule when
trying to set the enforcement timer for an expired job (without setting the
timer), and the second simply removes the BUG_ON (as there's a reasonable
situation where a timer can be set for a task that just ran out of budget,
even though the task is preemptible.)  A more complicated fix would be to
check for exhausted jobs when pulling off the ready queue, but that would
require a change in each scheduler.

These patches do appear to solve the particular issue I was having.

-Jeremy Erickson
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