Re: [RFC] sched/fair: Use wake_q length as a hint for wake_wide

[Brendan Jackman <brendan.jackman@arm.com>]

There has been a bit of discussion on this RFC, but before I do any
more work I'd really like your input on the basic idea.

Does the approach of feeding outside info like wake_q length into the
scheduler's decisions seem basically acceptable?

Cheers,
Brendan


On Fri, Aug 11 2017 at 09:45, Brendan Jackman wrote:
> This patch adds a parameter to select_task_rq, sibling_count_hint
> allowing the caller, where it has this information, to inform the
> sched_class the number of tasks that are being woken up as part of
> the same event.
>
> The wake_q mechanism is one case where this information is available.
>
> select_task_rq_fair can then use the information to detect that it
> needs to widen the search space for task placement in order to avoid
> overloading the last-level cache domain's CPUs.
>
>                                * * *
>
> The reason I am investigating this change is the following use case
> on ARM big.LITTLE (asymmetrical CPU capacity): 1 task per CPU, which
> all repeatedly do X amount of work then
> pthread_barrier_wait (i.e. sleep until the last task finishes its X
> and hits the barrier). On big.LITTLE, the tasks which get a "big" CPU
> finish faster, and then those CPUs pull over the tasks that are still
> running:
>
>      v CPU v           ->time->
>
>                     -------------
>    0  (big)         11111  /333
>                     -------------
>    1  (big)         22222   /444|
>                     -------------
>    2  (LITTLE)      333333/
>                     -------------
>    3  (LITTLE)      444444/
>                     -------------
>
> Now when task 4 hits the barrier (at |) and wakes the others up,
> there are 4 tasks with prev_cpu=<big> and 0 tasks with
> prev_cpu=<little>. want_affine therefore means that we'll only look
> in CPUs 0 and 1 (sd_llc), so tasks will be unnecessarily coscheduled
> on the bigs until the next load balance, something like this:
>
>      v CPU v           ->time->
>
>                     ------------------------
>    0  (big)         11111  /333  31313\33333
>                     ------------------------
>    1  (big)         22222   /444|424\4444444
>                     ------------------------
>    2  (LITTLE)      333333/          \222222
>                     ------------------------
>    3  (LITTLE)      444444/            \1111
>                     ------------------------
>                                  ^^^
>                            underutilization
>
> So, I'm trying to get want_affine = 0 for these tasks.
>
> I don't _think_ any incarnation of the wakee_flips mechanism can help
> us here because which task is waker and which tasks are wakees
> generally changes with each iteration.
>
> However pthread_barrier_wait (or more accurately FUTEX_WAKE) has the
> nice property that we know exactly how many tasks are being woken, so
> we can cheat.
>
> It might be a disadvantage that we "widen" _every_ task that's woken in
> an event, while select_idle_sibling would work fine for the first
> sd_llc_size - 1 tasks.
>
> IIUC, if wake_affine() behaves correctly this trick wouldn't be
> necessary on SMP systems, so it might be best guarded by the presence
> of SD_ASYM_CPUCAPACITY?
>
>                                * * *
>
> Final note..
>
> In order to observe "perfect" behaviour for this use case, I also had
> to disable the TTWU_QUEUE sched feature. Suppose during the wakeup
> above we are working through the work queue and have placed tasks 3
> and 2, and are about to place task 1:
>
>      v CPU v           ->time->
>
>                     --------------
>    0  (big)         11111  /333  3
>                     --------------
>    1  (big)         22222   /444|4
>                     --------------
>    2  (LITTLE)      333333/      2
>                     --------------
>    3  (LITTLE)      444444/          <- Task 1 should go here
>                     --------------
>
> If TTWU_QUEUE is enabled, we will not yet have enqueued task
> 2 (having instead sent a reschedule IPI) or attached its load to CPU
> 2. So we are likely to also place task 1 on cpu 2. Disabling
> TTWU_QUEUE means that we enqueue task 2 before placing task 1,
> solving this issue. TTWU_QUEUE is there to minimise rq lock
> contention, and I guess that this contention is less of an issue on
> big.LITTLE systems since they have relatively few CPUs, which
> suggests the trade-off makes sense here.
>
> Signed-off-by: Brendan Jackman <brendan.jackman@arm.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Josef Bacik <josef@toxicpanda.com>
> Cc: Joel Fernandes <joelaf@google.com>
> Cc: Mike Galbraith <efault@gmx.de>
> Cc: Matt Fleming <matt@codeblueprint.co.uk>
> ---
>  include/linux/sched/wake_q.h |  2 ++
>  kernel/sched/core.c          | 34 +++++++++++++++++++++++-----------
>  kernel/sched/deadline.c      |  3 ++-
>  kernel/sched/fair.c          | 17 +++++++++++------
>  kernel/sched/idle_task.c     |  3 ++-
>  kernel/sched/rt.c            |  3 ++-
>  kernel/sched/sched.h         |  3 ++-
>  kernel/sched/stop_task.c     |  3 ++-
>  8 files changed, 46 insertions(+), 22 deletions(-)
>
> diff --git a/include/linux/sched/wake_q.h b/include/linux/sched/wake_q.h
> index d03d8a9047dc..607a888eb35b 100644
> --- a/include/linux/sched/wake_q.h
> +++ b/include/linux/sched/wake_q.h
> @@ -33,6 +33,7 @@
>  struct wake_q_head {
>  	struct wake_q_node *first;
>  	struct wake_q_node **lastp;
> +	int count;
>  };
>
>  #define WAKE_Q_TAIL ((struct wake_q_node *) 0x01)
> @@ -44,6 +45,7 @@ static inline void wake_q_init(struct wake_q_head *head)
>  {
>  	head->first = WAKE_Q_TAIL;
>  	head->lastp = &head->first;
> +	head->count = 0;
>  }
>
>  extern void wake_q_add(struct wake_q_head *head,
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 0869b20fba81..ddf9257b1467 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -438,6 +438,8 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
>  	if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
>  		return;
>
> +	head->count++;
> +
>  	get_task_struct(task);
>
>  	/*
> @@ -447,6 +449,10 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task)
>  	head->lastp = &node->next;
>  }
>
> +static int
> +try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags,
> +	       int sibling_count_hint);
> +
>  void wake_up_q(struct wake_q_head *head)
>  {
>  	struct wake_q_node *node = head->first;
> @@ -461,10 +467,10 @@ void wake_up_q(struct wake_q_head *head)
>  		task->wake_q.next = NULL;
>
>  		/*
> -		 * wake_up_process() implies a wmb() to pair with the queueing
> +		 * try_to_wake_up() implies a wmb() to pair with the queueing
>  		 * in wake_q_add() so as not to miss wakeups.
>  		 */
> -		wake_up_process(task);
> +		try_to_wake_up(task, TASK_NORMAL, 0, head->count);
>  		put_task_struct(task);
>  	}
>  }
> @@ -1527,12 +1533,14 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
>   * The caller (fork, wakeup) owns p->pi_lock, ->cpus_allowed is stable.
>   */
>  static inline
> -int select_task_rq(struct task_struct *p, int cpu, int sd_flags, int wake_flags)
> +int select_task_rq(struct task_struct *p, int cpu, int sd_flags, int wake_flags,
> +		   int sibling_count_hint)
>  {
>  	lockdep_assert_held(&p->pi_lock);
>
>  	if (p->nr_cpus_allowed > 1)
> -		cpu = p->sched_class->select_task_rq(p, cpu, sd_flags, wake_flags);
> +		cpu = p->sched_class->select_task_rq(p, cpu, sd_flags, wake_flags,
> +						     sibling_count_hint);
>  	else
>  		cpu = cpumask_any(&p->cpus_allowed);
>
> @@ -1944,6 +1952,8 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
>   * @p: the thread to be awakened
>   * @state: the mask of task states that can be woken
>   * @wake_flags: wake modifier flags (WF_*)
> + * @sibling_count_hint: A hint at the number of threads that are being woken up
> + *                      in this event.
>   *
>   * If (@state & @p->state) @p->state = TASK_RUNNING.
>   *
> @@ -1956,7 +1966,8 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
>   *	   %false otherwise.
>   */
>  static int
> -try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> +try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags,
> +	       int sibling_count_hint)
>  {
>  	unsigned long flags;
>  	int cpu, success = 0;
> @@ -2042,7 +2053,8 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  		atomic_dec(&task_rq(p)->nr_iowait);
>  	}
>
> -	cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);
> +	cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags,
> +			     sibling_count_hint);
>  	if (task_cpu(p) != cpu) {
>  		wake_flags |= WF_MIGRATED;
>  		set_task_cpu(p, cpu);
> @@ -2130,13 +2142,13 @@ static void try_to_wake_up_local(struct task_struct *p, struct rq_flags *rf)
>   */
>  int wake_up_process(struct task_struct *p)
>  {
> -	return try_to_wake_up(p, TASK_NORMAL, 0);
> +	return try_to_wake_up(p, TASK_NORMAL, 0, 1);
>  }
>  EXPORT_SYMBOL(wake_up_process);
>
>  int wake_up_state(struct task_struct *p, unsigned int state)
>  {
> -	return try_to_wake_up(p, state, 0);
> +	return try_to_wake_up(p, state, 0, 1);
>  }
>
>  /*
> @@ -2442,7 +2454,7 @@ void wake_up_new_task(struct task_struct *p)
>  	 * Use __set_task_cpu() to avoid calling sched_class::migrate_task_rq,
>  	 * as we're not fully set-up yet.
>  	 */
> -	__set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0));
> +	__set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0, 1));
>  #endif
>  	rq = __task_rq_lock(p, &rf);
>  	update_rq_clock(rq);
> @@ -2893,7 +2905,7 @@ void sched_exec(void)
>  	int dest_cpu;
>
>  	raw_spin_lock_irqsave(&p->pi_lock, flags);
> -	dest_cpu = p->sched_class->select_task_rq(p, task_cpu(p), SD_BALANCE_EXEC, 0);
> +	dest_cpu = p->sched_class->select_task_rq(p, task_cpu(p), SD_BALANCE_EXEC, 0, 1);
>  	if (dest_cpu == smp_processor_id())
>  		goto unlock;
>
> @@ -3582,7 +3594,7 @@ asmlinkage __visible void __sched preempt_schedule_irq(void)
>  int default_wake_function(wait_queue_entry_t *curr, unsigned mode, int wake_flags,
>  			  void *key)
>  {
> -	return try_to_wake_up(curr->private, mode, wake_flags);
> +	return try_to_wake_up(curr->private, mode, wake_flags, 1);
>  }
>  EXPORT_SYMBOL(default_wake_function);
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 755bd3f1a1a9..69a9dd407267 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1516,7 +1516,8 @@ static void yield_task_dl(struct rq *rq)
>  static int find_later_rq(struct task_struct *task);
>
>  static int
> -select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)
> +select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags,
> +		  int sibling_count_hint)
>  {
>  	struct task_struct *curr;
>  	struct rq *rq;
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c95880e216f6..0a9d706b62bf 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5332,15 +5332,18 @@ static void record_wakee(struct task_struct *p)
>   * whatever is irrelevant, spread criteria is apparent partner count exceeds
>   * socket size.
>   */
> -static int wake_wide(struct task_struct *p)
> +static int wake_wide(struct task_struct *p, int sibling_count_hint)
>  {
>  	unsigned int master = current->wakee_flips;
>  	unsigned int slave = p->wakee_flips;
> -	int factor = this_cpu_read(sd_llc_size);
> +	int llc_size = this_cpu_read(sd_llc_size);
> +
> +	if (sibling_count_hint >= llc_size)
> +		return 1;
>
>  	if (master < slave)
>  		swap(master, slave);
> -	if (slave < factor || master < slave * factor)
> +	if (slave < llc_size || master < slave * llc_size)
>  		return 0;
>  	return 1;
>  }
> @@ -5869,7 +5872,8 @@ static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
>   * preempt must be disabled.
>   */
>  static int
> -select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags)
> +select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags,
> +		    int sibling_count_hint)
>  {
>  	struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;
>  	int cpu = smp_processor_id();
> @@ -5879,8 +5883,9 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
>
>  	if (sd_flag & SD_BALANCE_WAKE) {
>  		record_wakee(p);
> -		want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
> -			      && cpumask_test_cpu(cpu, &p->cpus_allowed);
> +		want_affine = !wake_wide(p, sibling_count_hint) &&
> +			      !wake_cap(p, cpu, prev_cpu) &&
> +			      cpumask_test_cpu(cpu, &p->cpus_allowed);
>  	}
>
>  	rcu_read_lock();
> diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
> index 0c00172db63e..3c343e055110 100644
> --- a/kernel/sched/idle_task.c
> +++ b/kernel/sched/idle_task.c
> @@ -9,7 +9,8 @@
>
>  #ifdef CONFIG_SMP
>  static int
> -select_task_rq_idle(struct task_struct *p, int cpu, int sd_flag, int flags)
> +select_task_rq_idle(struct task_struct *p, int cpu, int sd_flag, int flags,
> +		    int sibling_count_hint)
>  {
>  	return task_cpu(p); /* IDLE tasks as never migrated */
>  }
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 45caf937ef90..b9937dccb8b3 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1387,7 +1387,8 @@ static void yield_task_rt(struct rq *rq)
>  static int find_lowest_rq(struct task_struct *task);
>
>  static int
> -select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
> +select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags,
> +		  int sibling_count_hint)
>  {
>  	struct task_struct *curr;
>  	struct rq *rq;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index eeef1a3086d1..56ae525618e9 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1419,7 +1419,8 @@ struct sched_class {
>  	void (*put_prev_task) (struct rq *rq, struct task_struct *p);
>
>  #ifdef CONFIG_SMP
> -	int  (*select_task_rq)(struct task_struct *p, int task_cpu, int sd_flag, int flags);
> +	int  (*select_task_rq)(struct task_struct *p, int task_cpu, int sd_flag, int flags,
> +			       int subling_count_hint);
>  	void (*migrate_task_rq)(struct task_struct *p);
>
>  	void (*task_woken) (struct rq *this_rq, struct task_struct *task);
> diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
> index 9f69fb630853..d0ce4fbb18ef 100644
> --- a/kernel/sched/stop_task.c
> +++ b/kernel/sched/stop_task.c
> @@ -11,7 +11,8 @@
>
>  #ifdef CONFIG_SMP
>  static int
> -select_task_rq_stop(struct task_struct *p, int cpu, int sd_flag, int flags)
> +select_task_rq_stop(struct task_struct *p, int cpu, int sd_flag, int flags,
> +		    int sibling_count_hint)
>  {
>  	return task_cpu(p); /* stop tasks as never migrate */
>  }