linux-kernel - Re: [PATCH 1/4] sched/rt: Sum number of all children tasks in hierarhy at rt_nr

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Message-Id: <2983961395143041@web25j.yandex.ru>
Date:	Tue, 18 Mar 2014 15:44:01 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Preeti Murthy <preeti.lkml@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/4] sched/rt: Sum number of all children tasks in hierarhy at rt_nr_running



18.03.2014, 15:08, "Preeti Murthy" <preeti.lkml@...il.com>:
> On Sat, Mar 15, 2014 at 3:44 AM, Kirill Tkhai <tkhai@...dex.ru> wrote:
>
>>  {inc,dec}_rt_tasks used to count entities which are directly queued
>>  on rt_rq. If an entity was not a task (i.e., it is some queue), its
>>  children were not counted.
>
> Its always the case that a task is queued right, never a sched entity?

With patch applied, when sched entity is group queue, we add number of
its child tasks instead of "1".

> When a task is queued, the nr_running of every rt_rq in the hierarchy
> of sched entities which are parents of this task is incremented by 1.

Only if they had not had a queued task before.

> Similar is with dequeue of a sched_entity. If the sched_entity has
> just 1 task on it, then its dequeued from its parent queue and its number
> decremented for every rq in the hierarchy. But you would
> never dequeue a sched_entity if it has more than 1 task in it. The
> granularity of enqueue and dequeue of sched_entities is one task
> at a time. You can extend this to enqueue and dequeue of a sched_entity
> only if it has just one task in its queue.

We do not queue entites, which are empty. In __enqueue_rt_entity():

if (group_rq && (rt_rq_throttled(group_rq) || !group_rq->rt_nr_running))
         return;

Where do you see a collision? Please explain, if so.

> Regards
> Preeti U Murthy
>
>>  There is no problem here, but now we want to count number of all tasks
>>  which are actually queued under the rt_rq in all the hierarhy (except
>>  throttled rt queues).
>>
>>  Empty queues are not able to be queued and all of the places, which
>>  use rt_nr_running, just compare it with zero, so we do not break
>>  anything here.
>>
>>  Signed-off-by: Kirill Tkhai <tkhai@...dex.ru>
>>  CC: Peter Zijlstra <peterz@...radead.org>
>>  CC: Ingo Molnar <mingo@...nel.org>
>>  ---
>>   kernel/sched/rt.c |   15 +++++++++++++--
>>   1 file changed, 13 insertions(+), 2 deletions(-)
>>
>>  diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>>  index d8cdf16..e4def13 100644
>>  --- a/kernel/sched/rt.c
>>  +++ b/kernel/sched/rt.c
>>  @@ -1045,12 +1045,23 @@ void dec_rt_group(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq) {}
>>   #endif /* CONFIG_RT_GROUP_SCHED */
>>
>>   static inline
>>  +unsigned int rt_se_nr_running(struct sched_rt_entity *rt_se)
>>  +{
>>  +       struct rt_rq *group_rq = group_rt_rq(rt_se);
>>  +
>>  +       if (group_rq)
>>  +               return group_rq->rt_nr_running;
>>  +       else
>>  +               return 1;
>>  +}
>>  +
>>  +static inline
>>   void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
>>   {
>>          int prio = rt_se_prio(rt_se);
>>
>>          WARN_ON(!rt_prio(prio));
>>  -       rt_rq->rt_nr_running++;
>>  +       rt_rq->rt_nr_running += rt_se_nr_running(rt_se);
>>
>>          inc_rt_prio(rt_rq, prio);
>>          inc_rt_migration(rt_se, rt_rq);
>>  @@ -1062,7 +1073,7 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
>>   {
>>          WARN_ON(!rt_prio(rt_se_prio(rt_se)));
>>          WARN_ON(!rt_rq->rt_nr_running);
>>  -       rt_rq->rt_nr_running--;
>>  +       rt_rq->rt_nr_running -= rt_se_nr_running(rt_se);
>>
>>          dec_rt_prio(rt_rq, rt_se_prio(rt_se));
>>          dec_rt_migration(rt_se, rt_rq);
>>
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