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SubjectRe: [PATCH v3 04/10] sched/fair: rework load_balance
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On 01/10/2019 10:14, Vincent Guittot wrote:
> On Mon, 30 Sep 2019 at 18:24, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>>
>> Hi Vincent,
>>
>> On 19/09/2019 09:33, Vincent Guittot wrote:

[...]

>>> @@ -7347,7 +7362,7 @@ static int detach_tasks(struct lb_env *env)
>>> {
>>> struct list_head *tasks = &env->src_rq->cfs_tasks;
>>> struct task_struct *p;
>>> - unsigned long load;
>>> + unsigned long util, load;
>>
>> Minor: Order by length or reduce scope to while loop ?
>
> I don't get your point here

Nothing dramatic here! Just

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d0c3aa1dc290..a08f342ead89 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7333,8 +7333,8 @@ static const unsigned int sched_nr_migrate_break = 32;
static int detach_tasks(struct lb_env *env)
{
struct list_head *tasks = &env->src_rq->cfs_tasks;
- struct task_struct *p;
unsigned long load, util;
+ struct task_struct *p;
int detached = 0;

lockdep_assert_held(&env->src_rq->lock);
or

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d0c3aa1dc290..4d1864d43ed7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7334,7 +7334,6 @@ static int detach_tasks(struct lb_env *env)
{
struct list_head *tasks = &env->src_rq->cfs_tasks;
struct task_struct *p;
- unsigned long load, util;
int detached = 0;

lockdep_assert_held(&env->src_rq->lock);
@@ -7343,6 +7342,8 @@ static int detach_tasks(struct lb_env *env)
return 0;

while (!list_empty(tasks)) {
+ unsigned long load, util;
+
/*

[...]

>>> @@ -8042,14 +8104,24 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>>> }
>>> }
>>>
>>> - /* Adjust by relative CPU capacity of the group */
>>> + /* Check if dst cpu is idle and preferred to this group */
>>
>> s/preferred to/preferred by ? or the preferred CPU of this group ?
>
> dst cpu doesn't belong to this group. We compare asym_prefer_cpu of
> this group vs dst_cpu which belongs to another group

Ah, in the sense of 'preferred over'. Got it now!

[...]

>>> + if (busiest->group_type == group_imbalanced) {
>>> + /*
>>> + * In the group_imb case we cannot rely on group-wide averages
>>> + * to ensure CPU-load equilibrium, try to move any task to fix
>>> + * the imbalance. The next load balance will take care of
>>> + * balancing back the system.
>>
>> balancing back ?
>
> In case of imbalance, we don't try to balance the system but only try
> to get rid of the pinned tasks problem. The system will still be
> unbalanced after the migration and the next load balance will take
> care of balancing the system

OK.

[...]

>>> /*
>>> - * Avg load of busiest sg can be less and avg load of local sg can
>>> - * be greater than avg load across all sgs of sd because avg load
>>> - * factors in sg capacity and sgs with smaller group_type are
>>> - * skipped when updating the busiest sg:
>>> + * Try to use spare capacity of local group without overloading it or
>>> + * emptying busiest
>>> */
>>> - if (busiest->group_type != group_misfit_task &&
>>> - (busiest->avg_load <= sds->avg_load ||
>>> - local->avg_load >= sds->avg_load)) {
>>> - env->imbalance = 0;
>>> + if (local->group_type == group_has_spare) {
>>> + if (busiest->group_type > group_fully_busy) {
>>
>> So this could be 'busiest->group_type == group_overloaded' here to match
>> the comment below? Since you handle group_misfit_task,
>> group_asym_packing, group_imbalanced above and return.
>
> This is just to be more robust in case some new states are added later

OK, although I doubt that additional states can be added easily w/o
carefully auditing the entire lb code ;-)

[...]

>>> + if (busiest->group_weight == 1 || sds->prefer_sibling) {
>>> + /*
>>> + * When prefer sibling, evenly spread running tasks on
>>> + * groups.
>>> + */
>>> + env->balance_type = migrate_task;
>>> + env->imbalance = (busiest->sum_h_nr_running - local->sum_h_nr_running) >> 1;
>>> + return;
>>> + }
>>> +
>>> + /*
>>> + * If there is no overload, we just want to even the number of
>>> + * idle cpus.
>>> + */
>>> + env->balance_type = migrate_task;
>>> + env->imbalance = max_t(long, 0, (local->idle_cpus - busiest->idle_cpus) >> 1);
>>
>> Why do we need a max_t(long, 0, ...) here and not for the 'if
>> (busiest->group_weight == 1 || sds->prefer_sibling)' case?
>
> For env->imbalance = (busiest->sum_h_nr_running - local->sum_h_nr_running) >> 1;
>
> either we have sds->prefer_sibling && busiest->sum_nr_running >
> local->sum_nr_running + 1

I see, this corresponds to

/* Try to move all excess tasks to child's sibling domain */
if (sds.prefer_sibling && local->group_type == group_has_spare &&
busiest->sum_h_nr_running > local->sum_h_nr_running + 1)
goto force_balance;
in find_busiest_group, I assume.

Haven't been able to recreate this yet on my arm64 platform since there
is no prefer_sibling and in case local and busiest have
group_type=group_has_spare they bailout in

if (busiest->group_type != group_overloaded &&
(env->idle == CPU_NOT_IDLE ||
local->idle_cpus <= (busiest->idle_cpus + 1)))
goto out_balanced;

[...]

>>> - if (busiest->group_type == group_overloaded &&
>>> - local->group_type == group_overloaded) {
>>> - load_above_capacity = busiest->sum_h_nr_running * SCHED_CAPACITY_SCALE;
>>> - if (load_above_capacity > busiest->group_capacity) {
>>> - load_above_capacity -= busiest->group_capacity;
>>> - load_above_capacity *= scale_load_down(NICE_0_LOAD);
>>> - load_above_capacity /= busiest->group_capacity;
>>> - } else
>>> - load_above_capacity = ~0UL;
>>> + if (local->group_type < group_overloaded) {
>>> + /*
>>> + * Local will become overloaded so the avg_load metrics are
>>> + * finally needed.
>>> + */
>>
>> How does this relate to the decision_matrix[local, busiest] (dm[])? E.g.
>> dm[overload, overload] == avg_load or dm[fully_busy, overload] == force.
>> It would be nice to be able to match all allowed fields of dm to code sections.
>
> decision_matrix describes how it decides between balanced or unbalanced.
> In case of dm[overload, overload], we use the avg_load to decide if it
> is balanced or not

OK, that's why you calculate sgs->avg_load in update_sg_lb_stats() only
for 'sgs->group_type == group_overloaded'.

> In case of dm[fully_busy, overload], the groups are unbalanced because
> fully_busy < overload and we force the balance. Then
> calculate_imbalance() uses the avg_load to decide how much will be
> moved

And in this case 'local->group_type < group_overloaded' in
calculate_imbalance(), 'local->avg_load' and 'sds->avg_load' have to be
calculated before using them in env->imbalance = min(...).

OK, got it now.

> dm[overload, overload]=force means that we force the balance and we
> will compute later the imbalance. avg_load may be used to calculate
> the imbalance
> dm[overload, overload]=avg_load means that we compare the avg_load to
> decide whether we need to balance load between groups
> dm[overload, overload]=nr_idle means that we compare the number of
> idle cpus to decide whether we need to balance. In fact this is no
> more true with patch 7 because we also take into account the number of
> nr_h_running when weight =1

This becomes clearer now ... slowly.

[...]

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