Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()

["Li, Aubrey" <aubrey.li@linux.intel.com>]

On 2020/10/20 9:43, Joel Fernandes (Google) wrote:
> From: Peter Zijlstra <peterz@infradead.org>
> 
> Because sched_class::pick_next_task() also implies
> sched_class::set_next_task() (and possibly put_prev_task() and
> newidle_balance) it is not state invariant. This makes it unsuitable
> for remote task selection.
> 
> Tested-by: Julien Desfossez <jdesfossez@digitalocean.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Vineeth Remanan Pillai <vpillai@digitalocean.com>
> Signed-off-by: Julien Desfossez <jdesfossez@digitalocean.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/sched/deadline.c  | 16 ++++++++++++++--
>  kernel/sched/fair.c      | 32 +++++++++++++++++++++++++++++++-
>  kernel/sched/idle.c      |  8 ++++++++
>  kernel/sched/rt.c        | 14 ++++++++++++--
>  kernel/sched/sched.h     |  3 +++
>  kernel/sched/stop_task.c | 13 +++++++++++--
>  6 files changed, 79 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 814ec49502b1..0271a7848ab3 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1848,7 +1848,7 @@ static struct sched_dl_entity *pick_next_dl_entity(struct rq *rq,
>  	return rb_entry(left, struct sched_dl_entity, rb_node);
>  }
>  
> -static struct task_struct *pick_next_task_dl(struct rq *rq)
> +static struct task_struct *pick_task_dl(struct rq *rq)
>  {
>  	struct sched_dl_entity *dl_se;
>  	struct dl_rq *dl_rq = &rq->dl;
> @@ -1860,7 +1860,18 @@ static struct task_struct *pick_next_task_dl(struct rq *rq)
>  	dl_se = pick_next_dl_entity(rq, dl_rq);
>  	BUG_ON(!dl_se);
>  	p = dl_task_of(dl_se);
> -	set_next_task_dl(rq, p, true);
> +
> +	return p;
> +}
> +
> +static struct task_struct *pick_next_task_dl(struct rq *rq)
> +{
> +	struct task_struct *p;
> +
> +	p = pick_task_dl(rq);
> +	if (p)
> +		set_next_task_dl(rq, p, true);
> +
>  	return p;
>  }
>  
> @@ -2517,6 +2528,7 @@ const struct sched_class dl_sched_class
>  
>  #ifdef CONFIG_SMP
>  	.balance		= balance_dl,
> +	.pick_task		= pick_task_dl,
>  	.select_task_rq		= select_task_rq_dl,
>  	.migrate_task_rq	= migrate_task_rq_dl,
>  	.set_cpus_allowed       = set_cpus_allowed_dl,
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index dbd9368a959d..bd6aed63f5e3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4450,7 +4450,7 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>  	 * Avoid running the skip buddy, if running something else can
>  	 * be done without getting too unfair.
>  	 */
> -	if (cfs_rq->skip == se) {
> +	if (cfs_rq->skip && cfs_rq->skip == se) {
>  		struct sched_entity *second;
>  
>  		if (se == curr) {
> @@ -6976,6 +6976,35 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
>  		set_last_buddy(se);
>  }
>  
> +#ifdef CONFIG_SMP
> +static struct task_struct *pick_task_fair(struct rq *rq)
> +{
> +	struct cfs_rq *cfs_rq = &rq->cfs;
> +	struct sched_entity *se;
> +
> +	if (!cfs_rq->nr_running)
> +		return NULL;
> +
> +	do {
> +		struct sched_entity *curr = cfs_rq->curr;
> +
> +		se = pick_next_entity(cfs_rq, NULL);
> +
> +		if (curr) {
> +			if (se && curr->on_rq)
> +				update_curr(cfs_rq);
> +
> +			if (!se || entity_before(curr, se))
> +				se = curr;
> +		}
> +
> +		cfs_rq = group_cfs_rq(se);
> +	} while (cfs_rq);
> +
> +	return task_of(se);
> +}
> +#endif

One of my machines hangs when I run uperf with only one message:
[  719.034962] BUG: kernel NULL pointer dereference, address: 0000000000000050

Then I replicated the problem on my another machine(no serial console),
here is the stack by manual copy.

Call Trace:
 pick_next_entity+0xb0/0x160
 pick_task_fair+0x4b/0x90
 __schedule+0x59b/0x12f0
 schedule_idle+0x1e/0x40
 do_idle+0x193/0x2d0
 cpu_startup_entry+0x19/0x20
 start_secondary+0x110/0x150
 secondary_startup_64_no_verify+0xa6/0xab

> +
>  struct task_struct *
>  pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  {
> @@ -11173,6 +11202,7 @@ const struct sched_class fair_sched_class
>  
>  #ifdef CONFIG_SMP
>  	.balance		= balance_fair,
> +	.pick_task		= pick_task_fair,
>  	.select_task_rq		= select_task_rq_fair,
>  	.migrate_task_rq	= migrate_task_rq_fair,
>  
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 8ce6e80352cf..ce7552c6bc65 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -405,6 +405,13 @@ static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool fir
>  	schedstat_inc(rq->sched_goidle);
>  }
>  
> +#ifdef CONFIG_SMP
> +static struct task_struct *pick_task_idle(struct rq *rq)
> +{
> +	return rq->idle;
> +}
> +#endif
> +
>  struct task_struct *pick_next_task_idle(struct rq *rq)
>  {
>  	struct task_struct *next = rq->idle;
> @@ -472,6 +479,7 @@ const struct sched_class idle_sched_class
>  
>  #ifdef CONFIG_SMP
>  	.balance		= balance_idle,
> +	.pick_task		= pick_task_idle,
>  	.select_task_rq		= select_task_rq_idle,
>  	.set_cpus_allowed	= set_cpus_allowed_common,
>  #endif
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index e57fca05b660..a5851c775270 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1624,7 +1624,7 @@ static struct task_struct *_pick_next_task_rt(struct rq *rq)
>  	return rt_task_of(rt_se);
>  }
>  
> -static struct task_struct *pick_next_task_rt(struct rq *rq)
> +static struct task_struct *pick_task_rt(struct rq *rq)
>  {
>  	struct task_struct *p;
>  
> @@ -1632,7 +1632,16 @@ static struct task_struct *pick_next_task_rt(struct rq *rq)
>  		return NULL;
>  
>  	p = _pick_next_task_rt(rq);
> -	set_next_task_rt(rq, p, true);
> +
> +	return p;
> +}
> +
> +static struct task_struct *pick_next_task_rt(struct rq *rq)
> +{
> +	struct task_struct *p = pick_task_rt(rq);
> +	if (p)
> +		set_next_task_rt(rq, p, true);
> +
>  	return p;
>  }
>  
> @@ -2443,6 +2452,7 @@ const struct sched_class rt_sched_class
>  
>  #ifdef CONFIG_SMP
>  	.balance		= balance_rt,
> +	.pick_task		= pick_task_rt,
>  	.select_task_rq		= select_task_rq_rt,
>  	.set_cpus_allowed       = set_cpus_allowed_common,
>  	.rq_online              = rq_online_rt,
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 587ebabebaff..54bfac702805 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1800,6 +1800,9 @@ struct sched_class {
>  
>  #ifdef CONFIG_SMP
>  	int (*balance)(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
> +
> +	struct task_struct * (*pick_task)(struct rq *rq);
> +
>  	int  (*select_task_rq)(struct task_struct *p, int task_cpu, int sd_flag, int flags);
>  	void (*migrate_task_rq)(struct task_struct *p, int new_cpu);
>  
> diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
> index 394bc8126a1e..8f92915dd95e 100644
> --- a/kernel/sched/stop_task.c
> +++ b/kernel/sched/stop_task.c
> @@ -34,15 +34,23 @@ static void set_next_task_stop(struct rq *rq, struct task_struct *stop, bool fir
>  	stop->se.exec_start = rq_clock_task(rq);
>  }
>  
> -static struct task_struct *pick_next_task_stop(struct rq *rq)
> +static struct task_struct *pick_task_stop(struct rq *rq)
>  {
>  	if (!sched_stop_runnable(rq))
>  		return NULL;
>  
> -	set_next_task_stop(rq, rq->stop, true);
>  	return rq->stop;
>  }
>  
> +static struct task_struct *pick_next_task_stop(struct rq *rq)
> +{
> +	struct task_struct *p = pick_task_stop(rq);
> +	if (p)
> +		set_next_task_stop(rq, p, true);
> +
> +	return p;
> +}
> +
>  static void
>  enqueue_task_stop(struct rq *rq, struct task_struct *p, int flags)
>  {
> @@ -124,6 +132,7 @@ const struct sched_class stop_sched_class
>  
>  #ifdef CONFIG_SMP
>  	.balance		= balance_stop,
> +	.pick_task		= pick_task_stop,
>  	.select_task_rq		= select_task_rq_stop,
>  	.set_cpus_allowed	= set_cpus_allowed_common,
>  #endif
>