Re: [PATCH 5/7] RT: Add support for low-priority wake-up topush_rt feature

[Gregory Haskins]

On Mon, 2007-10-15 at 14:05 -0400, Steven Rostedt wrote:
> --
> On Fri, 12 Oct 2007, Gregory Haskins wrote:
> 
> > There are three events that require consideration for redistributing RT
> > tasks:
> >
> > 1) When one or more higher-priority tasks preempts a lower-one from a
> >    RQ
> > 2) When a lower-priority task is woken up on a RQ
> > 3) When a RQ downgrades its current priority
> >
> > Steve Rostedt's push_rt patch addresses (1).  It hooks in right after
> > a new task has been switched-in.  If this was the result of an RT
> > preemption, or if more than one task was awoken at the same time, we
> > can try to push some of those other tasks away.
> >
> > This patch addresses (2).  When we wake up a task, we check to see
> > if it would preempt the current task on the queue.  If it will not, we
> > attempt to find a better suited CPU (e.g. one running something lower
> > priority than the task being woken) and try to activate the task there.
> >
> > Finally, we have (3).  In theory, we only need to balance_rt_tasks() if
> > the following conditions are met:
> >    1) One or more CPUs are in overload, AND
> >    2) We are about to switch to a task that lowers our priority.
> >
> > (3) will be addressed in a later patch.
> >
> > Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
> > ---
> >
> >  kernel/sched.c |   68 ++++++++++++++++++++++++++++++++++----------------------
> >  1 files changed, 41 insertions(+), 27 deletions(-)
> >
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 62f9f0b..3c71156 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -1628,6 +1628,12 @@ out:
> >  	return ret;
> >  }
> >
> > +/* Push all tasks that we can to other CPUs */
> > +static void push_rt_tasks(struct rq *this_rq)
> > +{
> > +	while (push_rt_task(this_rq));
> 
> Loop conditions like this must be written as:
> 
>    while (push_rt_task(this_rq))
> 	;
> 
> So we don't accidently put something inside the loop if we forget to add
> the semicolon, like:
> 
>    while (push_rt_task(this_rq)
> 
>    do_something_not_expected_to_loop();
> 
> Of course you end your function after that and thus we would get an
> compile error if the semicolon were to be missing. But we might add
> code afterwards.

I'm not sure I really get what the difference is, but I don't feel
strongly one way or the other.  I can make that change.


> 
> > +}
> > +
> >  /*
> >   * Pull RT tasks from other CPUs in the RT-overload
> >   * case. Interrupts are disabled, local rq is locked.
> > @@ -1988,7 +1994,33 @@ out_set_cpu:
> >  		this_cpu = smp_processor_id();
> >  		cpu = task_cpu(p);
> >  	}
> > -
> > +
> > +	/*
> > +	 * If a newly woken up RT task cannot preempt the
> > +	 * current (RT) task (on a target runqueue) then try
> > +	 * to find another CPU it can preempt:
> > +	 */
> > +	if (rt_task(p) && !TASK_PREEMPTS_CURR(p, rq)) {
> > +		cpumask_t cpu_mask;
> > +		cpus_and(cpu_mask, cpu_online_map, p->cpus_allowed);
> 
> Hmm, maybe I should put that mask into the find_lowest_cpu function.
> Of course I changed this a little in my last patch.

If you have an update, send it along and I will rebase.


> 
> > +
> > +		new_cpu = find_lowest_cpu(&cpu_mask, p, rq);
> > +		if ((new_cpu != -1) && (new_cpu != cpu)) {
> > +			set_task_cpu(p, new_cpu);
> > +			spin_unlock(&rq->lock);
> > +
> > +			/* The new lock was already acquired in find_lowest */
> > +			rq = cpu_rq(new_cpu);
> > +			old_state = p->state;
> > +			if (!(old_state & state))
> > +				goto out;
> > +			if (p->se.on_rq)
> > +				goto out_running;
> > +
> > +			this_cpu = smp_processor_id();
> 
> Could we have preempted to get a new this_cpu?

That was a leftover from before we had the double_lock inside the search
function.  I will clean this up.

> 
> > +			cpu = task_cpu(p);
> > +		}
> > +	}
> >  out_activate:
> >  #endif /* CONFIG_SMP */
> >  	update_rq_clock(rq);
> > @@ -2002,30 +2034,13 @@ out_activate:
> >  	 * to find another CPU it can preempt:
> >  	 */
> >  	if (rt_task(p) && !TASK_PREEMPTS_CURR(p, rq)) {
> > -		struct rq *this_rq = cpu_rq(this_cpu);
> >  		/*
> > -		 * Special-case: the task on this CPU can be
> > -		 * preempted. In that case there's no need to
> > -		 * trigger reschedules on other CPUs, we can
> > -		 * mark the current task for reschedule.
> > -		 *
> > -		 * (Note that it's safe to access this_rq without
> > -		 * extra locking in this particular case, because
> > -		 * we are on the current CPU.)
> > +		 * FIXME: Do we still need to do this here anymore, or
> > +		 * does the preemption-check above suffice.  The path
> > +		 * that makes my head hurt is when we have the
> > +		 * task_running->out_activate path
> >  		 */
> > -		if (TASK_PREEMPTS_CURR(p, this_rq))
> > -			set_tsk_need_resched(this_rq->curr);
> > -		else
> > -			/*
> > -			 * Neither the intended target runqueue
> > -			 * nor the current CPU can take this task.
> > -			 * Trigger a reschedule on all other CPUs
> > -			 * nevertheless, maybe one of them can take
> > -			 * this task:
> > -			 */
> > -			smp_send_reschedule_allbutself_cpumask(p->cpus_allowed);
> > -
> > -		schedstat_inc(this_rq, rto_wakeup);
> > +		push_rt_tasks(rq);
> 
> I think the question is, doesn't this make the above not needed? The
> push_rt_tasks should do what the previous condition did.
> 
> Maybe I'm missing something.

Well, only that it has a few efficiency related advantages (1) to doing
this check before the activate() call.  You are correct that either
place would yield correct behavior.

(1) -> We can save the overhead of an unnecessary activate/deactivate
cycle, and avoid placing the system (even if only briefly) into overload
which will potentially affect other CPUs if they happen to call
_schedule() during the window.


> 
> >  	} else {
> >  		/*
> >  		 * Sync wakeups (i.e. those types of wakeups where the waker
> > @@ -2360,13 +2375,12 @@ static inline void finish_task_switch(struct rq *rq, struct task_struct *prev)
> >  	 * the lock was owned by prev, we need to release it
> >  	 * first via finish_lock_switch and then reaquire it.
> >  	 */
> > -	if (unlikely(rt_task(current))) {
> > +	if (unlikely(rq->rt_nr_running > 1)) {
> 
> Heh, I guess that would work.
> 
> -- Steve
> 
> >  		spin_lock(&rq->lock);
> > -		/* push_rt_task will return true if it moved an RT */
> > -		while (push_rt_task(rq))
> > -			;
> > +		push_rt_tasks(rq);
> >  		spin_unlock(&rq->lock);
> >  	}
> > +
> >  #endif
> >  	fire_sched_in_preempt_notifiers(current);
> >  	trace_stop_sched_switched(current);
> >
> >
> >

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