Re: [RFC PATCH v3 07/16] sched: Allow put_prev_task() to drop rq->lock

[mark gross <mgross@linux.intel.com>]

On Wed, May 29, 2019 at 08:36:43PM +0000, Vineeth Remanan Pillai wrote:
> From: Peter Zijlstra <peterz@infradead.org>
> 
> Currently the pick_next_task() loop is convoluted and ugly because of
> how it can drop the rq->lock and needs to restart the picking.
> 
> For the RT/Deadline classes, it is put_prev_task() where we do
> balancing, and we could do this before the picking loop. Make this
> possible.

Maybe explain why adding strtu rq_flags pointers to the function call supports
the above commit coment.  

--mark

> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/sched/core.c      |  2 +-
>  kernel/sched/deadline.c  | 14 +++++++++++++-
>  kernel/sched/fair.c      |  2 +-
>  kernel/sched/idle.c      |  2 +-
>  kernel/sched/rt.c        | 14 +++++++++++++-
>  kernel/sched/sched.h     |  4 ++--
>  kernel/sched/stop_task.c |  2 +-
>  7 files changed, 32 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 32ea79fb8d29..9dfa0c53deb3 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5595,7 +5595,7 @@ static void calc_load_migrate(struct rq *rq)
>  		atomic_long_add(delta, &calc_load_tasks);
>  }
>  
> -static void put_prev_task_fake(struct rq *rq, struct task_struct *prev)
> +static void put_prev_task_fake(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  {
>  }
>  
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index c02b3229e2c3..45425f971eec 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1772,13 +1772,25 @@ pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  	return p;
>  }
>  
> -static void put_prev_task_dl(struct rq *rq, struct task_struct *p)
> +static void put_prev_task_dl(struct rq *rq, struct task_struct *p, struct rq_flags *rf)
>  {
>  	update_curr_dl(rq);
>  
>  	update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 1);
>  	if (on_dl_rq(&p->dl) && p->nr_cpus_allowed > 1)
>  		enqueue_pushable_dl_task(rq, p);
> +
> +	if (rf && !on_dl_rq(&p->dl) && need_pull_dl_task(rq, p)) {
> +		/*
> +		 * This is OK, because current is on_cpu, which avoids it being
> +		 * picked for load-balance and preemption/IRQs are still
> +		 * disabled avoiding further scheduler activity on it and we've
> +		 * not yet started the picking loop.
> +		 */
> +		rq_unpin_lock(rq, rf);
> +		pull_dl_task(rq);
> +		rq_repin_lock(rq, rf);
> +	}
>  }
>  
>  /*
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 49707b4797de..8e3eb243fd9f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7110,7 +7110,7 @@ done: __maybe_unused;
>  /*
>   * Account for a descheduled task:
>   */
> -static void put_prev_task_fair(struct rq *rq, struct task_struct *prev)
> +static void put_prev_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  {
>  	struct sched_entity *se = &prev->se;
>  	struct cfs_rq *cfs_rq;
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index dd64be34881d..1b65a4c3683e 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -373,7 +373,7 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
>  	resched_curr(rq);
>  }
>  
> -static void put_prev_task_idle(struct rq *rq, struct task_struct *prev)
> +static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  {
>  }
>  
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index adec98a94f2b..51ee87c5a28a 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1593,7 +1593,7 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  	return p;
>  }
>  
> -static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
> +static void put_prev_task_rt(struct rq *rq, struct task_struct *p, struct rq_flags *rf)
>  {
>  	update_curr_rt(rq);
>  
> @@ -1605,6 +1605,18 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
>  	 */
>  	if (on_rt_rq(&p->rt) && p->nr_cpus_allowed > 1)
>  		enqueue_pushable_task(rq, p);
> +
> +	if (rf && !on_rt_rq(&p->rt) && need_pull_rt_task(rq, p)) {
> +		/*
> +		 * This is OK, because current is on_cpu, which avoids it being
> +		 * picked for load-balance and preemption/IRQs are still
> +		 * disabled avoiding further scheduler activity on it and we've
> +		 * not yet started the picking loop.
> +		 */
> +		rq_unpin_lock(rq, rf);
> +		pull_rt_task(rq);
> +		rq_repin_lock(rq, rf);
> +	}
>  }
>  
>  #ifdef CONFIG_SMP
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index bfcbcbb25646..4cbe2bef92e4 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1675,7 +1675,7 @@ struct sched_class {
>  	struct task_struct * (*pick_next_task)(struct rq *rq,
>  					       struct task_struct *prev,
>  					       struct rq_flags *rf);
> -	void (*put_prev_task)(struct rq *rq, struct task_struct *p);
> +	void (*put_prev_task)(struct rq *rq, struct task_struct *p, struct rq_flags *rf);
>  	void (*set_next_task)(struct rq *rq, struct task_struct *p);
>  
>  #ifdef CONFIG_SMP
> @@ -1721,7 +1721,7 @@ struct sched_class {
>  static inline void put_prev_task(struct rq *rq, struct task_struct *prev)
>  {
>  	WARN_ON_ONCE(rq->curr != prev);
> -	prev->sched_class->put_prev_task(rq, prev);
> +	prev->sched_class->put_prev_task(rq, prev, NULL);
>  }
>  
>  static inline void set_next_task(struct rq *rq, struct task_struct *next)
> diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
> index 47a3d2a18a9a..8f414018d5e0 100644
> --- a/kernel/sched/stop_task.c
> +++ b/kernel/sched/stop_task.c
> @@ -59,7 +59,7 @@ static void yield_task_stop(struct rq *rq)
>  	BUG(); /* the stop task should never yield, its pointless. */
>  }
>  
> -static void put_prev_task_stop(struct rq *rq, struct task_struct *prev)
> +static void put_prev_task_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  {
>  	struct task_struct *curr = rq->curr;
>  	u64 delta_exec;
> -- 
> 2.17.1
>