Re: [PATCH -tip 22/32] sched: Split the cookie and setup per-task cookie on fork

[Balbir Singh <bsingharora@gmail.com>]

On Tue, Nov 17, 2020 at 06:19:52PM -0500, Joel Fernandes (Google) wrote:
> In order to prevent interference and clearly support both per-task and CGroup
> APIs, split the cookie into 2 and allow it to be set from either per-task, or
> CGroup API. The final cookie is the combined value of both and is computed when
> the stop-machine executes during a change of cookie.
> 
> Also, for the per-task cookie, it will get weird if we use pointers of any
> emphemeral objects. For this reason, introduce a refcounted object who's sole
> purpose is to assign unique cookie value by way of the object's pointer.
> 
> While at it, refactor the CGroup code a bit. Future patches will introduce more
> APIs and support.
> 
> Reviewed-by: Josh Don <joshdon@google.com>
> Tested-by: Julien Desfossez <jdesfossez@digitalocean.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  include/linux/sched.h |   2 +
>  kernel/sched/core.c   | 241 ++++++++++++++++++++++++++++++++++++++++--
>  kernel/sched/debug.c  |   4 +
>  3 files changed, 236 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index a60868165590..c6a3b0fa952b 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -688,6 +688,8 @@ struct task_struct {
>  #ifdef CONFIG_SCHED_CORE
>  	struct rb_node			core_node;
>  	unsigned long			core_cookie;
> +	unsigned long			core_task_cookie;
> +	unsigned long			core_group_cookie;
>  	unsigned int			core_occupation;
>  #endif
>  
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b99a7493d590..7ccca355623a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -346,11 +346,14 @@ void sched_core_put(void)
>  	mutex_unlock(&sched_core_mutex);
>  }
>  
> +static int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2);
> +
>  #else /* !CONFIG_SCHED_CORE */
>  
>  static inline void sched_core_enqueue(struct rq *rq, struct task_struct *p) { }
>  static inline void sched_core_dequeue(struct rq *rq, struct task_struct *p) { }
>  static bool sched_core_enqueued(struct task_struct *task) { return false; }
> +static int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2) { }
>  
>  #endif /* CONFIG_SCHED_CORE */
>  
> @@ -4032,6 +4035,20 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
>  #endif
>  #ifdef CONFIG_SCHED_CORE
>  	RB_CLEAR_NODE(&p->core_node);
> +
> +	/*
> +	 * Tag child via per-task cookie only if parent is tagged via per-task
> +	 * cookie. This is independent of, but can be additive to the CGroup tagging.
> +	 */
> +	if (current->core_task_cookie) {
> +
> +		/* If it is not CLONE_THREAD fork, assign a unique per-task tag. */
> +		if (!(clone_flags & CLONE_THREAD)) {
> +			return sched_core_share_tasks(p, p);
> +               }
> +		/* Otherwise share the parent's per-task tag. */
> +		return sched_core_share_tasks(p, current);
> +	}
>  #endif
>  	return 0;
>  }
> @@ -9731,6 +9748,217 @@ static u64 cpu_rt_period_read_uint(struct cgroup_subsys_state *css,
>  #endif /* CONFIG_RT_GROUP_SCHED */
>  
>  #ifdef CONFIG_SCHED_CORE
> +/*
> + * A simple wrapper around refcount. An allocated sched_core_cookie's
> + * address is used to compute the cookie of the task.
> + */
> +struct sched_core_cookie {
> +	refcount_t refcnt;
> +};
> +
> +/*
> + * sched_core_tag_requeue - Common helper for all interfaces to set a cookie.
> + * @p: The task to assign a cookie to.
> + * @cookie: The cookie to assign.
> + * @group: is it a group interface or a per-task interface.
> + *
> + * This function is typically called from a stop-machine handler.

Can you clarify if it is typically or always, what are the implications for
locking?

> + */
> +void sched_core_tag_requeue(struct task_struct *p, unsigned long cookie, bool group)
> +{
> +	if (!p)
> +		return;
> +
> +	if (group)
> +		p->core_group_cookie = cookie;
> +	else
> +		p->core_task_cookie = cookie;
> +
> +	/* Use up half of the cookie's bits for task cookie and remaining for group cookie. */
> +	p->core_cookie = (p->core_task_cookie <<
> +				(sizeof(unsigned long) * 4)) + p->core_group_cookie;
> +

Always use masks to ensure this fits in the space we have, should we be concerned about
overflows and the potential for collision of cookie values?

> +	if (sched_core_enqueued(p)) {
> +		sched_core_dequeue(task_rq(p), p);
> +		if (!p->core_task_cookie)
> +			return;
> +	}
> +
> +	if (sched_core_enabled(task_rq(p)) &&
> +			p->core_cookie && task_on_rq_queued(p))
> +		sched_core_enqueue(task_rq(p), p);
> +}
> +
> +/* Per-task interface */
> +static unsigned long sched_core_alloc_task_cookie(void)
> +{
> +	struct sched_core_cookie *ptr =
> +		kmalloc(sizeof(struct sched_core_cookie), GFP_KERNEL);
> +
> +	if (!ptr)
> +		return 0;
> +	refcount_set(&ptr->refcnt, 1);
> +
> +	/*
> +	 * NOTE: sched_core_put() is not done by put_task_cookie(). Instead, it
> +	 * is done after the stopper runs.
> +	 */
> +	sched_core_get();
> +	return (unsigned long)ptr;
> +}
> +
> +static bool sched_core_get_task_cookie(unsigned long cookie)
> +{
> +	struct sched_core_cookie *ptr = (struct sched_core_cookie *)cookie;
> +
> +	/*
> +	 * NOTE: sched_core_put() is not done by put_task_cookie(). Instead, it
> +	 * is done after the stopper runs.
> +	 */
> +	sched_core_get();
> +	return refcount_inc_not_zero(&ptr->refcnt);
> +}
> +
> +static void sched_core_put_task_cookie(unsigned long cookie)
> +{
> +	struct sched_core_cookie *ptr = (struct sched_core_cookie *)cookie;
> +
> +	if (refcount_dec_and_test(&ptr->refcnt))
> +		kfree(ptr);
> +}
> +
> +struct sched_core_task_write_tag {
> +	struct task_struct *tasks[2];
> +	unsigned long cookies[2];
> +};

Use a better name instead of 2?

> +
> +/*
> + * Ensure that the task has been requeued. The stopper ensures that the task cannot
> + * be migrated to a different CPU while its core scheduler queue state is being updated.
> + * It also makes sure to requeue a task if it was running actively on another CPU.
> + */
> +static int sched_core_task_join_stopper(void *data)
> +{
> +	struct sched_core_task_write_tag *tag = (struct sched_core_task_write_tag *)data;
> +	int i;
> +
> +	for (i = 0; i < 2; i++)

Use ARRAY_SIZE(cookies) if you have to or ARRAY_SIZE(tasks)

> +		sched_core_tag_requeue(tag->tasks[i], tag->cookies[i], false /* !group */);
> +
> +	return 0;
> +}
> +
> +static int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2)
> +{

Can you please explain how t1 and t2 are related, there is a table below, but
I don't understand case#2, where the cookies get reset, is t2 the core leader
and t2 leads what t1 and t2 collectively get?

May be just called t2 as parent?

> +	struct sched_core_task_write_tag wr = {}; /* for stop machine. */
> +	bool sched_core_put_after_stopper = false;
> +	unsigned long cookie;
> +	int ret = -ENOMEM;
> +
> +	mutex_lock(&sched_core_mutex);
> +
> +	/*
> +	 * NOTE: sched_core_get() is done by sched_core_alloc_task_cookie() or
> +	 *       sched_core_put_task_cookie(). However, sched_core_put() is done
> +	 *       by this function *after* the stopper removes the tasks from the
> +	 *       core queue, and not before. This is just to play it safe.
> +	 */
> +	if (t2 == NULL) {
> +		if (t1->core_task_cookie) {
> +			sched_core_put_task_cookie(t1->core_task_cookie);
> +			sched_core_put_after_stopper = true;
> +			wr.tasks[0] = t1; /* Keep wr.cookies[0] reset for t1. */
> +		}
> +	} else if (t1 == t2) {
> +		/* Assign a unique per-task cookie solely for t1. */
> +
> +		cookie = sched_core_alloc_task_cookie();
> +		if (!cookie)
> +			goto out_unlock;
> +
> +		if (t1->core_task_cookie) {
> +			sched_core_put_task_cookie(t1->core_task_cookie);
> +			sched_core_put_after_stopper = true;
> +		}
> +		wr.tasks[0] = t1;
> +		wr.cookies[0] = cookie;
> +	} else
> +	/*
> +	 * 		t1		joining		t2
> +	 * CASE 1:
> +	 * before	0				0
> +	 * after	new cookie			new cookie
> +	 *
> +	 * CASE 2:
> +	 * before	X (non-zero)			0
> +	 * after	0				0
> +	 *
> +	 * CASE 3:
> +	 * before	0				X (non-zero)
> +	 * after	X				X
> +	 *
> +	 * CASE 4:
> +	 * before	Y (non-zero)			X (non-zero)
> +	 * after	X				X
> +	 */
> +	if (!t1->core_task_cookie && !t2->core_task_cookie) {
> +		/* CASE 1. */
> +		cookie = sched_core_alloc_task_cookie();
> +		if (!cookie)
> +			goto out_unlock;
> +
> +		/* Add another reference for the other task. */
> +		if (!sched_core_get_task_cookie(cookie)) {
> +			ret = -EINVAL;
> +			goto out_unlock;
> +		}
> +
> +		wr.tasks[0] = t1;
> +		wr.tasks[1] = t2;
> +		wr.cookies[0] = wr.cookies[1] = cookie;
> +
> +	} else if (t1->core_task_cookie && !t2->core_task_cookie) {
> +		/* CASE 2. */
> +		sched_core_put_task_cookie(t1->core_task_cookie);
> +		sched_core_put_after_stopper = true;
> +
> +		wr.tasks[0] = t1; /* Reset cookie for t1. */
> +
> +	} else if (!t1->core_task_cookie && t2->core_task_cookie) {
> +		/* CASE 3. */
> +		if (!sched_core_get_task_cookie(t2->core_task_cookie)) {
> +			ret = -EINVAL;
> +			goto out_unlock;
> +		}
> +
> +		wr.tasks[0] = t1;
> +		wr.cookies[0] = t2->core_task_cookie;
> +
> +	} else {
> +		/* CASE 4. */
> +		if (!sched_core_get_task_cookie(t2->core_task_cookie)) {
> +			ret = -EINVAL;
> +			goto out_unlock;
> +		}
> +		sched_core_put_task_cookie(t1->core_task_cookie);
> +		sched_core_put_after_stopper = true;
> +
> +		wr.tasks[0] = t1;
> +		wr.cookies[0] = t2->core_task_cookie;
> +	}
> +
> +	stop_machine(sched_core_task_join_stopper, (void *)&wr, NULL);
> +
> +	if (sched_core_put_after_stopper)
> +		sched_core_put();
> +
> +	ret = 0;
> +out_unlock:
> +	mutex_unlock(&sched_core_mutex);
> +	return ret;
> +}
> +
> +/* CGroup interface */
>  static u64 cpu_core_tag_read_u64(struct cgroup_subsys_state *css, struct cftype *cft)
>  {
>  	struct task_group *tg = css_tg(css);
> @@ -9761,18 +9989,9 @@ static int __sched_write_tag(void *data)
>  	 * when we set cgroup tag to 0 when the loop is done below.
>  	 */
>  	while ((p = css_task_iter_next(&it))) {
> -		p->core_cookie = !!val ? (unsigned long)tg : 0UL;
> -
> -		if (sched_core_enqueued(p)) {
> -			sched_core_dequeue(task_rq(p), p);
> -			if (!p->core_cookie)
> -				continue;
> -		}
> -
> -		if (sched_core_enabled(task_rq(p)) &&
> -		    p->core_cookie && task_on_rq_queued(p))
> -			sched_core_enqueue(task_rq(p), p);
> +		unsigned long cookie = !!val ? (unsigned long)tg : 0UL;
>  
> +		sched_core_tag_requeue(p, cookie, true /* group */);
>  	}
>  	css_task_iter_end(&it);
>  
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 60a922d3f46f..8c452b8010ad 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -1024,6 +1024,10 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
>  		__PS("clock-delta", t1-t0);
>  	}
>  
> +#ifdef CONFIG_SCHED_CORE
> +	__PS("core_cookie", p->core_cookie);
> +#endif
> +
>  	sched_show_numa(p, m);
>  }
>

Balbir Singh.