RE: Mainline kernel OLTP performance update

[Chris Mason]

On Mon, 2009-01-19 at 13:37 -0500, Steven Rostedt wrote:
> (added Rusty)
> 
> On Mon, 2009-01-19 at 13:04 -0500, Chris Mason wrote:
> > 
> > I think the -rt version of check_preempt_equal_prio has gotten much more
> > expensive since 2.6.24.
> > 
> > I'm sure these changes were made for good reasons, and this workload may
> > not be a good reason to change it back.  But, what does the patch below
> > do to performance on 2.6.29-rcX?
> > 
> > -chris
> > 
> > diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
> > index 954e1a8..bbe3492 100644
> > --- a/kernel/sched_rt.c
> > +++ b/kernel/sched_rt.c
> > @@ -842,6 +842,7 @@ static void check_preempt_curr_rt(struct rq *rq,
> > struct task_struct *p, int sync
> >  		resched_task(rq->curr);
> >  		return;
> >  	}
> > +	return;
> >  
> >  #ifdef CONFIG_SMP
> >  	/*
> 
> That should not cause much of a problem if the scheduling task is not
> pinned to an CPU. But!!!!!
> 
> A recent change makes it expensive:


> +       if (!alloc_cpumask_var(&mask, GFP_ATOMIC))
>                 return;

> check_preempt_equal_prio is in a scheduling hot path!!!!!
> 
> WTF are we allocating there for?

I wasn't actually looking at the cost of the checks, even though they do
look higher (if they are using CONFIG_CPUMASK_OFFSTACK anyway).

The 2.6.24 code would trigger a rescheduling interrupt only when the
prio of the inbound task was higher than the running task.

This workload has a large number of equal priority rt tasks that are not
bound to a single CPU, and so I think it should trigger more
preempts/reschedules with the today's check_preempt_equal_prio().

-chris


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