Re: [RFC 2/4] sched/core: Set nr_lat_sensitive counter at various scheduler entry/exit points

[Parth Shah]

Hi Pavan,

Thanks for going through this patch-set.

On 5/8/20 2:03 PM, Pavan Kondeti wrote:
> Hi Parth,
> 
> On Thu, May 07, 2020 at 07:07:21PM +0530, Parth Shah wrote:
>> Monitor tasks at:
>> 1. wake_up_new_task() - forked tasks
>>
>> 2. set_task_cpu() - task migrations, Load balancer
>>
>> 3. __sched_setscheduler() - set/unset latency_nice value
>> Increment the nr_lat_sensitive count on the CPU with task marked with
>> latency_nice == -20.
>> Similarly, decrement the nr_lat_sensitive counter upon re-marking the task
>> with >-20 latency_nice task.
>>
>> 4. finish_task_switch() - dying task
>>
> 
> 
>> Signed-off-by: Parth Shah <parth@xxxxxxxxxxxxx>
>> ---
>>  kernel/sched/core.c  | 30 ++++++++++++++++++++++++++++--
>>  kernel/sched/sched.h |  5 +++++
>>  2 files changed, 33 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 2d8b76f41d61..ad396c36eba6 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -1744,6 +1744,11 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
>>  	trace_sched_migrate_task(p, new_cpu);
>>  
>>  	if (task_cpu(p) != new_cpu) {
>> +		if (task_is_lat_sensitive(p)) {
>> +			per_cpu(nr_lat_sensitive, task_cpu(p))--;
>> +			per_cpu(nr_lat_sensitive, new_cpu)++;
>> +		}
>> +
> 
> Since we can come here without rq locks, there is a possibility
> of a race and incorrect updates can happen. Since the counters
> are used to prevent C-states, we don't want that to happen.

I did tried using task_lock(p) wherever we do change refcount and when
latency_nice value is set. There I was using nr_lat_sensitive with atomic_t
type.

After lots of thinking to optimize it and thinking that we anyways hold rq
lock, I thought of not using any lock here and see if scheduler community
has well known solution for this :-)

But in brief, using atomic_t nr_lat_sensitive and task_lock(p) when changin
refcount should solve problem, right?

If you or anyone have solution for this kind of pattern, then that surely
will be helpful.

> 
>>  		if (p->sched_class->migrate_task_rq)
>>  			p->sched_class->migrate_task_rq(p, new_cpu);
>>  		p->se.nr_migrations++;
>> @@ -2947,6 +2952,7 @@ void wake_up_new_task(struct task_struct *p)
>>  {
>>  	struct rq_flags rf;
>>  	struct rq *rq;
>> +	int target_cpu = 0;
>>  
>>  	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
>>  	p->state = TASK_RUNNING;
>> @@ -2960,9 +2966,17 @@ void wake_up_new_task(struct task_struct *p)
>>  	 * as we're not fully set-up yet.
>>  	 */
>>  	p->recent_used_cpu = task_cpu(p);
>> -	__set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0));
>> +	target_cpu = select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0);
>> +	__set_task_cpu(p, target_cpu);
>> +
> 
> The target_cpu variable can be eliminated by using task_cpu(p) directly
> in the below update.

Right. Will change it thus saving one diff line.

> 
>>  #endif
>>  	rq = __task_rq_lock(p, &rf);
>> +
>> +#ifdef CONFIG_SMP
>> +	if (task_is_lat_sensitive(p))
>> +		per_cpu(nr_lat_sensitive, target_cpu)++;
>> +#endif
>> +
> 
> Is the SMP check intentional? In some parts of this patch, updates to
> nr_lat_sensitive are done without SMP checks. For example,
> finish_task_switch() below.

No. just forget to remove. I will remove SMP checks in next revision.

> 
>>  	update_rq_clock(rq);
>>  	post_init_entity_util_avg(p);
>>  
>> @@ -3248,6 +3262,9 @@ static struct rq *finish_task_switch(struct task_struct *prev)
>>  		if (prev->sched_class->task_dead)
>>  			prev->sched_class->task_dead(prev);
>>  
>> +		if (task_is_lat_sensitive(prev))
>> +			per_cpu(nr_lat_sensitive, prev->cpu)--;
>> +
>>  		/*
>>  		 * Remove function-return probe instances associated with this
>>  		 * task and put them back on the free list.
>> @@ -4732,8 +4749,17 @@ static void __setscheduler_params(struct task_struct *p,
>>  	p->normal_prio = normal_prio(p);
>>  	set_load_weight(p, true);
>>  
>> -	if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE)
>> +	if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE) {
>> +		if (p->state != TASK_DEAD &&
>> +		    attr->sched_latency_nice != p->latency_nice) {
>> +			if (attr->sched_latency_nice == MIN_LATENCY_NICE)
>> +				per_cpu(nr_lat_sensitive, task_cpu(p))++;
>> +			else if (task_is_lat_sensitive(p))
>> +				per_cpu(nr_lat_sensitive, task_cpu(p))--;
>> +		}
>> +
>>  		p->latency_nice = attr->sched_latency_nice;
>> +	}
>>  }
> 
> There is a potential race here due to which we can mess up the refcount.
> 
> - A latency sensitive task is marked TASK_DEAD
> <snip>
> - sched_setattr() called on the task to clear the latency nice. Since
> we check the task state here, we skip the decrement.
> - The task is finally context switched out and we skip the decrement again
> since it is not a latency senstivie task.

if task is already marked TASK_DEAD then we should have already decremented
its refcount in finish_task_switch().
am I missing something?

> 
>>  
>>  /* Actually do priority change: must hold pi & rq lock. */
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 5c41020c530e..56f885e37451 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -211,6 +211,11 @@ static inline int task_has_dl_policy(struct task_struct *p)
>>  	return dl_policy(p->policy);
>>  }
>>  
>> +static inline int task_is_lat_sensitive(struct task_struct *p)
>> +{
>> +	return p->latency_nice == MIN_LATENCY_NICE;
>> +}
>> +
>>  #define cap_scale(v, s) ((v)*(s) >> SCHED_CAPACITY_SHIFT)
>>  
>>  /*
>> -- 
>> 2.17.2
>>
> 
> Thanks,
> Pavan
> 

Thanks,
Parth