linux-kernel - Re: [PATCH v5 03/12] sched: fix avg

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Message-ID: <1409848011.31585.35.camel@schen9-desk2.jf.intel.com>
Date:	Thu, 04 Sep 2014 09:26:51 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Rik van Riel <riel@...hat.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v5 03/12] sched: fix avg_load computation

On Thu, 2014-09-04 at 09:17 +0200, Vincent Guittot wrote:
> On 4 September 2014 01:43, Tim Chen <tim.c.chen@...ux.intel.com> wrote:
> > On Wed, 2014-09-03 at 13:09 +0200, Vincent Guittot wrote:
> >> On 30 August 2014 14:00, Preeti U Murthy <preeti@...ux.vnet.ibm.com> wrote:
> >> > Hi Vincent,
> >> >
> >> > On 08/26/2014 04:36 PM, Vincent Guittot wrote:
> >> >> The computation of avg_load and avg_load_per_task should only takes into
> >> >> account the number of cfs tasks. The non cfs task are already taken into
> >> >> account by decreasing the cpu's capacity and they will be tracked in the
> >> >> CPU's utilization (group_utilization) of the next patches
> >> >>
> >> >> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> >> >> ---
> >> >>  kernel/sched/fair.c | 4 ++--
> >> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >> >>
> >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> >> index 87b9dc7..b85e9f7 100644
> >> >> --- a/kernel/sched/fair.c
> >> >> +++ b/kernel/sched/fair.c
> >> >> @@ -4092,7 +4092,7 @@ static unsigned long capacity_of(int cpu)
> >> >>  static unsigned long cpu_avg_load_per_task(int cpu)
> >> >>  {
> >> >>       struct rq *rq = cpu_rq(cpu);
> >> >> -     unsigned long nr_running = ACCESS_ONCE(rq->nr_running);
> >> >> +     unsigned long nr_running = ACCESS_ONCE(rq->cfs.h_nr_running);
> >> >>       unsigned long load_avg = rq->cfs.runnable_load_avg;
> >> >>
> >> >>       if (nr_running)
> >> >> @@ -5985,7 +5985,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
> >> >>                       load = source_load(i, load_idx);
> >> >>
> >> >>               sgs->group_load += load;
> >> >> -             sgs->sum_nr_running += rq->nr_running;
> >> >> +             sgs->sum_nr_running += rq->cfs.h_nr_running;
> >> >>
> >> >>               if (rq->nr_running > 1)
> >> >>                       *overload = true;
> >> >>
> >> >
> >> > Why do we probe rq->nr_running while we do load balancing? Should not we
> >> > be probing cfs_rq->nr_running instead? We are interested after all in
> >> > load balancing fair tasks right? The reason I ask this is, I was
> >> > wondering if we need to make the above similar change in more places in
> >> > load balancing.
> >>
> >> Hi Preeti,
> >>
> >> Yes, we should probably the test   rq->cfs.h_nr_running > 0 before
> >> setting overload.
> >>
> >
> > The overload indicator is used for knowing when we can totally avoid
> > load balancing to a cpu that is about to go idle.
> > We can avoid load balancing when no cpu has more than 1 task.  So if you
> > have say just one fair task and multiple deadline tasks on a cpu,
> > and another cpu about to go idle, you should turn on normal load
> > balancing in the idle path by setting overload to true.
> 
> The newly idle load balancing can only affect CFS tasks so triggering
> a load_balance because a cpu is overloaded by rt tasks only, will not
> change anything.
> 
> >
> > So setting overload should be set based on rq->nr_running and not on
> > rq->cfs.h_nr_running.
> 
> We should probably use both values like below
> 
> if ((rq->nr_running > 1) && ( rq->cfs.h_nr_running > 0))

Yes, this modification is the correct one that takes care of the
condition I objected to previously.

Thanks.

Tim


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