Re: [RFC PATCH 10/16] sched: Trivial forced-newidle balancer(Internet mail)

["benbjiang(蒋彪)" <benbjiang@tencent.com>]

> On Jul 20, 2020, at 4:03 PM, Li, Aubrey <aubrey.li@linux.intel.com> wrote:
> 
> On 2020/7/20 15:23, benbjiang(蒋彪) wrote:
>> Hi, 
>> 
>>> On Jul 20, 2020, at 2:06 PM, Li, Aubrey <aubrey.li@linux.intel.com <mailto:aubrey.li@linux.intel.com>> wrote:
>>> 
>>> On 2020/7/20 12:06, benbjiang(蒋彪) wrote:
>>>> Hi,
>>>> 
>>>>> On Jul 1, 2020, at 5:32 AM, Vineeth Remanan Pillai <vpillai@digitalocean.com <mailto:vpillai@digitalocean.com>> wrote:
>>>>> 
>>>>> From: Peter Zijlstra <peterz@infradead.org <mailto:peterz@infradead.org>>
>>>>> 
>>>>> When a sibling is forced-idle to match the core-cookie; search for
>>>>> matching tasks to fill the core.
>>>>> 
>>>>> rcu_read_unlock() can incur an infrequent deadlock in
>>>>> sched_core_balance(). Fix this by using the RCU-sched flavor instead.
>>>>> 
>>>>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org <mailto:peterz@infradead.org>>
>>>>> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org <mailto:joel@joelfernandes.org>>
>>>>> Acked-by: Paul E. McKenney <paulmck@kernel.org <mailto:paulmck@kernel.org>>
>>>>> ---
>>>>> include/linux/sched.h |   1 +
>>>>> kernel/sched/core.c   | 131 +++++++++++++++++++++++++++++++++++++++++-
>>>>> kernel/sched/idle.c   |   1 +
>>>>> kernel/sched/sched.h  |   6 ++
>>>>> 4 files changed, 138 insertions(+), 1 deletion(-)
>>>>> 
>>>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>>>>> index 3c8dcc5ff039..4f9edf013df3 100644
>>>>> --- a/include/linux/sched.h
>>>>> +++ b/include/linux/sched.h
>>>>> @@ -688,6 +688,7 @@ struct task_struct {
>>>>> #ifdef CONFIG_SCHED_CORE
>>>>> struct rb_nodecore_node;
>>>>> unsigned longcore_cookie;
>>>>> +unsigned intcore_occupation;
>>>>> #endif
>>>>> 
>>>>> #ifdef CONFIG_CGROUP_SCHED
>>>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>>>> index 4d6d6a678013..fb9edb09ead7 100644
>>>>> --- a/kernel/sched/core.c
>>>>> +++ b/kernel/sched/core.c
>>>>> @@ -201,6 +201,21 @@ static struct task_struct *sched_core_find(struct rq *rq, unsigned long cookie)
>>>>> return match;
>>>>> }
>>>>> 
>>>>> +static struct task_struct *sched_core_next(struct task_struct *p, unsigned long cookie)
>>>>> +{
>>>>> +struct rb_node *node = &p->core_node;
>>>>> +
>>>>> +node = rb_next(node);
>>>>> +if (!node)
>>>>> +return NULL;
>>>>> +
>>>>> +p = container_of(node, struct task_struct, core_node);
>>>>> +if (p->core_cookie != cookie)
>>>>> +return NULL;
>>>>> +
>>>>> +return p;
>>>>> +}
>>>>> +
>>>>> /*
>>>>> * The static-key + stop-machine variable are needed such that:
>>>>> *
>>>>> @@ -4233,7 +4248,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>>>>> struct task_struct *next, *max = NULL;
>>>>> const struct sched_class *class;
>>>>> const struct cpumask *smt_mask;
>>>>> -int i, j, cpu;
>>>>> +int i, j, cpu, occ = 0;
>>>>> bool need_sync;
>>>>> 
>>>>> if (!sched_core_enabled(rq))
>>>>> @@ -4332,6 +4347,9 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>>>>> goto done;
>>>>> }
>>>>> 
>>>>> +if (!is_idle_task(p))
>>>>> +occ++;
>>>>> +
>>>>> rq_i->core_pick = p;
>>>>> 
>>>>> /*
>>>>> @@ -4357,6 +4375,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>>>>> 
>>>>> cpu_rq(j)->core_pick = NULL;
>>>>> }
>>>>> +occ = 1;
>>>>> goto again;
>>>>> } else {
>>>>> /*
>>>>> @@ -4393,6 +4412,8 @@ next_class:;
>>>>> if (is_idle_task(rq_i->core_pick) && rq_i->nr_running)
>>>>> rq_i->core_forceidle = true;
>>>>> 
>>>>> +rq_i->core_pick->core_occupation = occ;
>>>>> +
>>>>> if (i == cpu)
>>>>> continue;
>>>>> 
>>>>> @@ -4408,6 +4429,114 @@ next_class:;
>>>>> return next;
>>>>> }
>>>>> 
>>>>> +static bool try_steal_cookie(int this, int that)
>>>>> +{
>>>>> +struct rq *dst = cpu_rq(this), *src = cpu_rq(that);
>>>>> +struct task_struct *p;
>>>>> +unsigned long cookie;
>>>>> +bool success = false;
>>>>> +
>>>>> +local_irq_disable();
>>>>> +double_rq_lock(dst, src);
>>>>> +
>>>>> +cookie = dst->core->core_cookie;
>>>>> +if (!cookie)
>>>>> +goto unlock;
>>>>> +
>>>>> +if (dst->curr != dst->idle)
>>>>> +goto unlock;
>>>>> +
>>>>> +p = sched_core_find(src, cookie);
>>>>> +if (p == src->idle)
>>>>> +goto unlock;
>>>>> +
>>>>> +do {
>>>>> +if (p == src->core_pick || p == src->curr)
>>>>> +goto next;
>>>>> +
>>>>> +if (!cpumask_test_cpu(this, &p->cpus_mask))
>>>>> +goto next;
>>>>> +
>>>>> +if (p->core_occupation > dst->idle->core_occupation)
>>>>> +goto next;
>>>>> +
>>>>> +p->on_rq = TASK_ON_RQ_MIGRATING;
>>>>> +deactivate_task(src, p, 0);
>>>>> +set_task_cpu(p, this);
>>>>> +activate_task(dst, p, 0);
>>>>> +p->on_rq = TASK_ON_RQ_QUEUED;
>>>>> +
>>>>> +resched_curr(dst);
>>>>> +
>>>>> +success = true;
>>>>> +break;
>>>>> +
>>>>> +next:
>>>>> +p = sched_core_next(p, cookie);
>>>>> +} while (p);
>>>>> +
>>>>> +unlock:
>>>>> +double_rq_unlock(dst, src);
>>>>> +local_irq_enable();
>>>>> +
>>>>> +return success;
>>>>> +}
>>>>> +
>>>>> +static bool steal_cookie_task(int cpu, struct sched_domain *sd)
>>>>> +{
>>>>> +int i;
>>>>> +
>>>>> +for_each_cpu_wrap(i, sched_domain_span(sd), cpu) {
>>>> Since (i == cpu) should be skipped, should we start iteration at cpu+1? like,
>>>> for_each_cpu_wrap(i, sched_domain_span(sd), cpu+1) {
>>>> …
>>>> }
>>>> In that way, we could avoid hitting following if(i == cpu) always.
>>> 
>>> IMHO, this won't work, as cpuid is not continuous.
>> Cpuid may be not continuous, but for_each_cpu_wrap() could cover the case, I think. :)
> 
> And for_each_cpu_wrap() will still wrap around and pick i == cpu, even though it starts
> from (cpu+1)...
> 
> Thanks,
> -Aubrey
Yep, but that’s the last choice, we may steal the right task in most cases without skipping. 
Just an option. :)

> 
>> 
>>> 
>>>>> +if (i == cpu)
>>>>> +continue;
>>>>> +
>>>>> +if (need_resched())
>>>>> +break;
>>>> Should we return true here to accelerate the breaking of sched_core_balance? 
>>>> Otherwise the breaking would be delayed to the next level sd iteration.
And could you have a look here. :)
Thanks.

Regards,
Jiang 

>>>>> +
>>>>> +if (try_steal_cookie(cpu, i))
>>>>> +return true;
>>>>> +}
>>>>> +
>>>>> +return false;
>>>>> +}
>>>>> +
>>>>> +static void sched_core_balance(struct rq *rq)
>>>>> +{
>>>>> +struct sched_domain *sd;
>>>>> +int cpu = cpu_of(rq);
>>>>> +
>>>>> +rcu_read_lock_sched();
>>>>> +raw_spin_unlock_irq(rq_lockp(rq));
>>>>> +for_each_domain(cpu, sd) {
>>>>> +if (!(sd->flags & SD_LOAD_BALANCE))
>>>>> +break;
>>>>> +
>>>>> +if (need_resched())
>>>>> +break;
>>>> If rescheded here, we missed the chance to do further forced-newidle balance, 
>>>> and the idle-core could be idle for a long time, because lacking of pulling chance.
>>>> Could it be possible to add a new forced-newidle balance chance in task_tick_idle?
>>>> which could make it more efficient.
>>> 
>>> This flag indicates there is another thread deserves to run, So I guess the core won't
>>> be idle for a long time.
>>> 
>>> Thanks,
>>> -Aubrey
>> Indeed, thanks for the explanation. :)
>> 
>>>> 
>>>>> +if (steal_cookie_task(cpu, sd))
>>>>> +break;
>>>>> +}
>>>>> +raw_spin_lock_irq(rq_lockp(rq));
>>>>> +rcu_read_unlock_sched();
>>>>> +}
>>>>> +
>>>>> +static DEFINE_PER_CPU(struct callback_head, core_balance_head);
>>>>> +
>>>>> +void queue_core_balance(struct rq *rq)
>>>>> +{
>>>>> +if (!sched_core_enabled(rq))
>>>>> +return;
>>>>> +
>>>>> +if (!rq->core->core_cookie)
>>>>> +return;
>>>>> +
>>>>> +if (!rq->nr_running) /* not forced idle */
>>>>> +return;
>>>>> +
>>>>> +queue_balance_callback(rq, &per_cpu(core_balance_head, rq->cpu), sched_core_balance);
>>>>> +}
>>>>> +
>>>>> #else /* !CONFIG_SCHED_CORE */
>>>>> 
>>>>> static struct task_struct *
>>>>> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
>>>>> index a8d40ffab097..dff6ba220ed7 100644
>>>>> --- a/kernel/sched/idle.c
>>>>> +++ b/kernel/sched/idle.c
>>>>> @@ -395,6 +395,7 @@ static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool fir
>>>>> {
>>>>> update_idle_core(rq);
>>>>> schedstat_inc(rq->sched_goidle);
>>>>> +queue_core_balance(rq);
>>>>> }
>>>>> 
>>>>> #ifdef CONFIG_SMP
>>>>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>>>>> index 293aa1ae0308..464559676fd2 100644
>>>>> --- a/kernel/sched/sched.h
>>>>> +++ b/kernel/sched/sched.h
>>>>> @@ -1089,6 +1089,8 @@ static inline raw_spinlock_t *rq_lockp(struct rq *rq)
>>>>> bool cfs_prio_less(struct task_struct *a, struct task_struct *b);
>>>>> void sched_core_adjust_sibling_vruntime(int cpu, bool coresched_enabled);
>>>>> 
>>>>> +extern void queue_core_balance(struct rq *rq);
>>>>> +
>>>>> #else /* !CONFIG_SCHED_CORE */
>>>>> 
>>>>> static inline bool sched_core_enabled(struct rq *rq)
>>>>> @@ -1101,6 +1103,10 @@ static inline raw_spinlock_t *rq_lockp(struct rq *rq)
>>>>> return &rq->__lock;
>>>>> }
>>>>> 
>>>>> +static inline void queue_core_balance(struct rq *rq)
>>>>> +{
>>>>> +}
>>>>> +
>>>>> #endif /* CONFIG_SCHED_CORE */
>>>>> 
>>>>> #ifdef CONFIG_SCHED_SMT
>>>>> -- 
>>>>> 2.17.1
>> 
> 
>