Re: [tip:sched/core] sched: Push put_prev_task() into pick_next_task( )

[Kirill Tkhai <tkhai@yandex.ru>]

12.02.2014, 18:06, "Peter Zijlstra" <peterz@infradead.org>:
> On Wed, Feb 12, 2014 at 11:00:53AM +0400, Kirill Tkhai wrote:
>
>>>  @@ -4748,7 +4743,7 @@ static void migrate_tasks(unsigned int dead_cpu)
>>>                   if (rq->nr_running == 1)
>>>                           break;
>>>
>>>  - next = pick_next_task(rq);
>>>  + next = pick_next_task(rq, NULL);
>>  pick_next_task() firstly checks for prev->sched_class, doesn't it ???
>>
>>  The same for pick_next_task_rt().
>
> OK, how about something like this?

Good for me.

Maybe pack prev->sched_class->put_prev_task() into inline function?

Kirill

> ---
> Subject: sched: Fix hotplug task migration
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Wed, 12 Feb 2014 10:49:30 +0100
>
> Dan Carpenter reported:
>
>>  kernel/sched/rt.c:1347 pick_next_task_rt() warn: variable dereferenced before check 'prev' (see line 1338)
>>  kernel/sched/deadline.c:1011 pick_next_task_dl() warn: variable dereferenced before check 'prev' (see line 1005)
>
> Kirill also spotted that migrate_tasks() will have an instant NULL
> deref because pick_next_task() will immediately deref prev.
>
> Instead of fixing all the corner cases because migrate_tasks() can
> pass in a NULL prev task in the unlikely case of hot-un-plug, provide
> a fake task such that we can remove all the NULL checks from the far
> more common paths.
>
> A further problem; not previously spotted; is that because we pushed
> pre_schedule() and idle_balance() into pick_next_task() we now need to
> avoid those getting called and pulling more tasks on our dying CPU.
>
> We avoid pull_{dl,rt}_task() by setting fake_task.prio to MAX_PRIO+1.
> We also note that since we call pick_next_task() exactly the amount of
> times we have runnable tasks present, we should never land in
> idle_balance().
>
> Fixes: 38033c37faab ("sched: Push down pre_schedule() and idle_balance()")
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Juri Lelli <juri.lelli@gmail.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Reported-by: Kirill Tkhai <tkhai@yandex.ru>
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> Link: http://lkml.kernel.org/r/20140212094930.GB3545@laptop.programming.kicks-ass.net
> ---
>  kernel/sched/core.c      |   18 +++++++++++++++++-
>  kernel/sched/deadline.c  |    3 +--
>  kernel/sched/fair.c      |    5 ++---
>  kernel/sched/idle_task.c |    3 +--
>  kernel/sched/rt.c        |    3 +--
>  kernel/sched/stop_task.c |    3 +--
>  6 files changed, 23 insertions(+), 12 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4681,6 +4681,22 @@ static void calc_load_migrate(struct rq
>                  atomic_long_add(delta, &calc_load_tasks);
>  }
>
> +static void put_prev_task_fake(struct rq *rq, struct task_struct *prev)
> +{
> +}
> +
> +static const struct sched_class fake_sched_class = {
> + .put_prev_task = put_prev_task_fake,
> +};
> +
> +static struct task_struct fake_task = {
> + /*
> + * Avoid pull_{rt,dl}_task()
> + */
> + .prio = MAX_PRIO + 1,
> + .sched_class = &fake_sched_class,
> +};
> +
>  /*
>   * Migrate all tasks from the rq, sleeping tasks will be migrated by
>   * try_to_wake_up()->select_task_rq().
> @@ -4721,7 +4737,7 @@ static void migrate_tasks(unsigned int d
>                  if (rq->nr_running == 1)
>                          break;
>
> - next = pick_next_task(rq, NULL);
> + next = pick_next_task(rq, &fake_task);
>                  BUG_ON(!next);
>                  next->sched_class->put_prev_task(rq, next);
>
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1008,8 +1008,7 @@ struct task_struct *pick_next_task_dl(st
>          if (unlikely(!dl_rq->dl_nr_running))
>                  return NULL;
>
> - if (prev)
> - prev->sched_class->put_prev_task(rq, prev);
> + prev->sched_class->put_prev_task(rq, prev);
>
>          dl_se = pick_next_dl_entity(rq, dl_rq);
>          BUG_ON(!dl_se);
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4690,7 +4690,7 @@ pick_next_task_fair(struct rq *rq, struc
>          if (!cfs_rq->nr_running)
>                  goto idle;
>
> - if (!prev || prev->sched_class != &fair_sched_class)
> + if (prev->sched_class != &fair_sched_class)
>                  goto simple;
>
>          /*
> @@ -4766,8 +4766,7 @@ pick_next_task_fair(struct rq *rq, struc
>          if (!cfs_rq->nr_running)
>                  goto idle;
>
> - if (prev)
> - prev->sched_class->put_prev_task(rq, prev);
> + prev->sched_class->put_prev_task(rq, prev);
>
>          do {
>                  se = pick_next_entity(cfs_rq, NULL);
> --- a/kernel/sched/idle_task.c
> +++ b/kernel/sched/idle_task.c
> @@ -26,8 +26,7 @@ static void check_preempt_curr_idle(stru
>  static struct task_struct *
>  pick_next_task_idle(struct rq *rq, struct task_struct *prev)
>  {
> - if (prev)
> - prev->sched_class->put_prev_task(rq, prev);
> + prev->sched_class->put_prev_task(rq, prev);
>
>          schedstat_inc(rq, sched_goidle);
>  #ifdef CONFIG_SMP
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1344,8 +1344,7 @@ pick_next_task_rt(struct rq *rq, struct
>          if (rt_rq_throttled(rt_rq))
>                  return NULL;
>
> - if (prev)
> - prev->sched_class->put_prev_task(rq, prev);
> + prev->sched_class->put_prev_task(rq, prev);
>
>          p = _pick_next_task_rt(rq);
>
> --- a/kernel/sched/stop_task.c
> +++ b/kernel/sched/stop_task.c
> @@ -31,8 +31,7 @@ pick_next_task_stop(struct rq *rq, struc
>          if (!stop || !stop->on_rq)
>                  return NULL;
>
> - if (prev)
> - prev->sched_class->put_prev_task(rq, prev);
> + prev->sched_class->put_prev_task(rq, prev);
>
>          stop->se.exec_start = rq_clock_task(rq);