Re: [PATCH] Repost sched-rt: Reduce rq lock contention by eliminating locking of non-feasible target

[Tim Chen]

On Thu, 2015-01-15 at 20:58 -0500, Steven Rostedt wrote:

> 
> Please add a comment here that says something like:
> 
> 		/*
> 		 * Don't bother moving it if the destination CPU is
> 		 * not running a lower priority task.
> 		 */
> 
Okay.  Updated in patch below.

> > -		if (target != -1)
> > +		if (target != -1 &&
> > +		    p->prio < cpu_rq(target)->rt.highest_prio.curr)
> >  			cpu = target;
> >  	}
> >  	rcu_read_unlock();
> > @@ -1613,6 +1614,12 @@ static struct rq *find_lock_lowest_rq(struct
> > task_struct *task, struct rq *rq) break;
> >  
> >  		lowest_rq = cpu_rq(cpu);
> > +		
> > +		if (lowest_rq->rt.highest_prio.curr <= task->prio) {
> > +		/* target rq has tasks of equal or higher priority,
> > try again */
> > +			lowest_rq = NULL;
> > +			continue;
> 
> This should just break out and not try again. The reason for the other
> try again is because of the double_lock which can release the locks
> which can cause a process waiting for the lock to sneak in and
> change the priorities. But this case, a try again is highly unlikely to
> do anything differently (no locks are released) and just waste cycles.

Agree.  Updated in updated patch below.

Thanks.

Tim

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