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SubjectRe: [RFC PATCH -rt] Priority preemption latency
Date
On Friday 09 June 2006 23:48, Ingo Molnar wrote:
> * Darren Hart <dvhltc@us.ibm.com> wrote:
> > @@ -1543,6 +1543,17 @@ static int try_to_wake_up(task_t *p, uns
> > }
> > }
> >
> > + /*
> > + * XXX Don't send RT task elsewhere unless it can preempt current
> > + * XXX on other CPU. Better yet would be for awakened RT tasks to
> > + * XXX examine this(and all other) CPU(s) to see what is the best
> > + * XXX fit. For example there is no check here to see if the
> > + * XXX currently running task can be preempted (which would be the
> > + * XXX ideal case).
> > + */
> > + if (rt_task(p) && !TASK_PREEMPTS_CURR(p, rq))
> > + goto out_set_cpu;
> > +
>
> Great testcase! Note that we already do RT-overload wakeups further
> below:
>
> /*
> * If a newly woken up RT task cannot preempt the
> * current (RT) task then try to find another
> * CPU it can preempt:
> */
> if (rt_task(p) && !TASK_PREEMPTS_CURR(p, rq)) {
> smp_send_reschedule_allbutself();
> rt_overload_wakeup++;
> }
>
> what i think happened is that the above logic misses the case you have
> described in detail, and doesnt mark the current CPU for rescheduling.
>
> I.e. it sends an IPI to all _other_ CPUs, including the 'wrong target' -
> but it doesnt mark the current CPU for reschedule - hence if the current
> CPU is the only right target we might fail to handle this task!
>
> could you try the (untested) patch below, does it solve your testcase
> too?

Thanks for the updated patch! It wouldn't quite build (proc_misc.c still
referenced the old rt_overload_* variables, fixup patch attached removing
those print statements). I have it running on a 4 way opteron box running
prio-preempt in a timed while loop, exiting only on failure. It's been
running fine for several minutes - usually fails in under a mintue. We'll
see how it's doing in the morning :-)

--
Darren Hart
IBM Linux Technology Center
Realtime Linux Team
diff -Nurp linux-2.6.16/fs/proc/proc_misc.c linux-2.6.16-fixup/fs/proc/proc_misc.c
--- linux-2.6.16/fs/proc/proc_misc.c 2006-06-10 16:48:23.000000000 -0700
+++ linux-2.6.16-fixup/fs/proc/proc_misc.c 2006-06-10 17:00:55.000000000 -0700
@@ -569,19 +569,11 @@ static int show_stat(struct seq_file *p,
{
unsigned long nr_uninterruptible_cpu(int cpu);
extern int pi_initialized;
- extern int rt_overload_schedule,
- rt_overload_wakeup, rt_overload_pulled;
unsigned long rt_nr_running_cpu(int cpu);
extern atomic_t rt_overload;

int i;

- seq_printf(p, "rt_overload_schedule: %d\n",
- rt_overload_schedule);
- seq_printf(p, "rt_overload_wakeup: %d\n",
- rt_overload_wakeup);
- seq_printf(p, "rt_overload_pulled: %d\n",
- rt_overload_pulled);
seq_printf(p, "pi_init: %d\n", pi_initialized);
seq_printf(p, "nr_running(): %ld\n",
nr_running());
@@ -593,8 +585,6 @@ static int show_stat(struct seq_file *p,
for_each_cpu(i)
seq_printf(p, "rt_nr_running(%d): %ld\n",
i, rt_nr_running_cpu(i));
- seq_printf(p, "rt_overload: %d\n", atomic_read(&rt_overload));
-
}
#endif
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