RE: Mainline kernel OLTP performance update

Steven Rostedt
2009-01-19T18:40:35Z

(added Rusty)

On Mon, 2009-01-19 at 13:04 -0500, Chris Mason wrote:
> On Thu, 2009-01-15 at 00:11 -0700, Ma, Chinang wrote:
> > >> > > > >
> > >> > > > > Linux OLTP Performance summary
> > >> > > > > Kernel# Speedup(x) Intr/s CtxSw/s us% sys% idle%
> > >iowait%
> > >> > > > > 2.6.24.2 1.000 21969 43425 76 24 0
> > >0
> > >> > > > > 2.6.27.2 0.973 30402 43523 74 25 0
> > >1
> > >> > > > > 2.6.29-rc1 0.965 30331 41970 74 26 0
> > >0
> > >> >
> > >> > > But the interrupt rate went through the roof.
> > >> >
> > >> > Yes. I forget why that was; I'll have to dig through my archives for
> > >> > that.
> > >>
> > >> Oh. I'd have thought that this alone could account for 3.5%.
>
> A later email indicated the reschedule interrupt count doubled since
> 2.6.24, and so I poked around a bit at the causes of resched_task.
>
> 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:

commit 24600ce89a819a8f2fb4fd69fd777218a82ade20
Author: Rusty Russell <rusty@rustcorp.com.au>
Date: Tue Nov 25 02:35:13 2008 +1030

sched: convert check_preempt_equal_prio to cpumask_var_t.

Impact: stack reduction for large NR_CPUS

which has:

static void check_preempt_equal_prio(struct rq *rq, struct task_struct
*p)
{
- cpumask_t mask;
+ cpumask_var_t mask;

if (rq->curr->rt.nr_cpus_allowed == 1)
return;

- if (p->rt.nr_cpus_allowed != 1
- && cpupri_find(&rq->rd->cpupri, p, &mask))
+ if (!alloc_cpumask_var(&mask, GFP_ATOMIC))
return;

check_preempt_equal_prio is in a scheduling hot path!!!!!

WTF are we allocating there for?

-- Steve