Hi PeterZ,

I just got this in my inbox and noticed I didn't adress it to anyone. I
meant to address it to you.

On Fri, Sep 29 2017 at 17:05, Brendan Jackman wrote:

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 */
}

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