On 02/24/2014 08:12 PM, Peter Zijlstra wrote:
[snip]
>>
>> ...what about move idle_balance() back to it's old position?
>
> I've always hated that, idle_balance() is very much a fair policy thing
> and shouldn't live in the core code.
>
>> pull_rt_task() logical could be after idle_balance() if still no FAIR
>> and DL, then go into the pick loop, that may could make things more
>> clean & clear, should we have a try?
>
> So the reason pull_{rt,dl}_task() is before idle_balance() is that we
> don't want to add the execution latency of idle_balance() to the rt/dl
> task pulling.
Yeah, that make sense, just wondering... since RT also has balance
stuff, may be we can use a new call back for each class in the old position?
The new idle_balance could like:
void idle_balance() {
for_each_class(class)
if class->idle_balance()
break
}
>
> Anyway, the below seems to work; it avoids playing tricks with the idle
> thread and instead uses a magic constant.
>
> The comparison should be faster too; seeing how we avoid dereferencing
> p->sched_class.
Great, it once appeared in my mind but you achieved this without new
parameter, now let's ignore my wondering above :)
Regards,
Michael Wang
>
> ---
> Subject: sched: Guarantee task priority in pick_next_task()
> From: Peter Zijlstra <pet...@infradead.org>
> Date: Fri Feb 14 12:25:08 CET 2014
>
> Michael spotted that the idle_balance() push down created a task
> priority problem.
>
> Previously, when we called idle_balance() before pick_next_task() it
> wasn't a problem when -- because of the rq->lock droppage -- an rt/dl
> task slipped in.
>
> Similarly for pre_schedule(), rt pre-schedule could have a dl task
> slip in.
>
> But by pulling it into the pick_next_task() loop, we'll not try a
> higher task priority again.
>
> Cure this by creating a re-start condition in pick_next_task(); and
> triggering this from pick_next_task_{rt,fair}().
>
> Fixes: 38033c37faab ("sched: Push down pre_schedule() and idle_balance()")
> Cc: Juri Lelli <juri.le...@gmail.com>
> Cc: Ingo Molnar <mi...@kernel.org>
> Cc: Steven Rostedt <rost...@goodmis.org>
> Reported-by: Michael Wang <wang...@linux.vnet.ibm.com>
> Signed-off-by: Peter Zijlstra <pet...@infradead.org>
> ---
> kernel/sched/core.c | 12 ++++++++----
> kernel/sched/fair.c | 13 ++++++++++++-
> kernel/sched/rt.c | 10 +++++++++-
> kernel/sched/sched.h | 5 +++++
> 4 files changed, 34 insertions(+), 6 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2586,24 +2586,28 @@ static inline void schedule_debug(struct
> static inline struct task_struct *
> pick_next_task(struct rq *rq, struct task_struct *prev)
> {
> - const struct sched_class *class;
> + const struct sched_class *class = &fair_sched_class;
> struct task_struct *p;
>
> /*
> * Optimization: we know that if all tasks are in
> * the fair class we can call that function directly:
> */
> - if (likely(prev->sched_class == &fair_sched_class &&
> + if (likely(prev->sched_class == class &&
> rq->nr_running == rq->cfs.h_nr_running)) {
> p = fair_sched_class.pick_next_task(rq, prev);
> - if (likely(p))
> + if (likely(p && p != RETRY_TASK))
> return p;
> }
>
> +again:
> for_each_class(class) {
> p = class->pick_next_task(rq, prev);
> - if (p)
> + if (p) {
> + if (unlikely(p == RETRY_TASK))
> + goto again;
> return p;
> + }
> }
>
> BUG(); /* the idle class will always have a runnable task */
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4687,6 +4687,7 @@ pick_next_task_fair(struct rq *rq, struc
> struct cfs_rq *cfs_rq = &rq->cfs;
> struct sched_entity *se;
> struct task_struct *p;
> + int new_tasks;
>
> again:
> #ifdef CONFIG_FAIR_GROUP_SCHED
> @@ -4785,7 +4786,17 @@ pick_next_task_fair(struct rq *rq, struc
> return p;
>
> idle:
> - if (idle_balance(rq)) /* drops rq->lock */
> + /*
> + * Because idle_balance() releases (and re-acquires) rq->lock, it is
> + * possible for any higher priority task to appear. In that case we
> + * must re-start the pick_next_entity() loop.
> + */
> + new_tasks = idle_balance(rq);
> +
> + if (rq->nr_running != rq->cfs.h_nr_running)
> + return RETRY_TASK;
> +
> + if (new_tasks)
> goto again;
>
> return NULL;
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1360,8 +1360,16 @@ pick_next_task_rt(struct rq *rq, struct
> struct task_struct *p;
> struct rt_rq *rt_rq = &rq->rt;
>
> - if (need_pull_rt_task(rq, prev))
> + if (need_pull_rt_task(rq, prev)) {
> pull_rt_task(rq);
> + /*
> + * pull_rt_task() can drop (and re-acquire) rq->lock; this
> + * means a dl task can slip in, in which case we need to
> + * re-start task selection.
> + */
> + if (unlikely(rq->dl.dl_nr_running))
> + return RETRY_TASK;
> + }
>
> if (!rt_rq->rt_nr_running)
> return NULL;
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1090,6 +1090,8 @@ static const u32 prio_to_wmult[40] = {
>
> #define DEQUEUE_SLEEP 1
>
> +#define RETRY_TASK ((void *)-1UL)
> +
> struct sched_class {
> const struct sched_class *next;
>
> @@ -1104,6 +1106,9 @@ struct sched_class {
> * It is the responsibility of the pick_next_task() method that will
> * return the next task to call put_prev_task() on the @prev task or
> * something equivalent.
> + *
> + * May return RETRY_TASK when it finds a higher prio class has runnable
> + * tasks.
> */
> struct task_struct * (*pick_next_task) (struct rq *rq,
> struct task_struct *prev);
> --
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