Re: sched: Consequences of integrating the Per Entity Load TrackingMetric into the Load Balancer

[Preeti U Murthy]

Hi Alex,

On 01/16/2013 07:38 PM, Alex Shi wrote:
> On 01/08/2013 04:41 PM, Preeti U Murthy wrote:
>> Hi Mike,
>>
>> Thank you very much for such a clear and comprehensive explanation.
>> So when I put together the problem and the proposed solution pieces in the current
>> scheduler scalability,the following was what I found:
>>
>> 1. select_idle_sibling() is needed as an agent to correctly find the right cpu for wake
>>    up tasks to go to."Correctly" would be to find an idle cpu at the lowest cost possible.
>> 2."Cost could be lowered" either by optimizing the order of searching for an idle cpu or
>>    restricting the search to a few cpus alone.
>> 3. The former has the problem that it would not prevent bouncing tasks all over the domain
>>    sharing an L3 cache,which could potentially affect the fast moving tasks.
>> 4. The latter has the problem that it is not aggressive enough in finding an idle cpu.
>>
>> This is some tangled problem,but I think the solution at best could be smoothed to a a flowchart.
>>
>>        STEP1                       STEP2                STEP3
>>  _____________________
>> |                     |
>> |See if the idle buddy|No    _________________  Yes   ________________
>> |is free at all sched |---->| Do we search the|----> |Optimized search|
>> |domains              |     |sched domains    |      |________________|
>> |_____________________|     |for an idle cpu  |                 |
>>           |Yes              |_________________|                \|/
>>          \|/                        |No: saturated     Return target cpu
>>         Return                     \|/     system
>>         cpu buddy                Return prev_cpu
>>
> 
> 
> 
> I re-written the patch as following. hackbench/aim9 doest show clean performance change.
> Actually we can get some profit. it also will be very slight. :) 
> BTW, it still need another patch before apply this. Just to show the logical.
> 
> ===========
> From 145ff27744c8ac04eda056739fe5aa907a00877e Mon Sep 17 00:00:00 2001
> From: Alex Shi <alex.shi@xxxxxxxxx>
> Date: Fri, 11 Jan 2013 16:49:03 +0800
> Subject: [PATCH 3/7] sched: select_idle_sibling optimization
> 
> Current logical in this function will insist to wake up the task in a
> totally idle group, otherwise it would rather back to previous cpu.

As Namhyung pointed out this could be the waking cpu as well.
> 
> The new logical will try to wake up the task on any idle cpu in the same
> cpu socket (in same sd_llc), while idle cpu in the smaller domain has
> higher priority.

Here is where the problem of large sockets come in.select_idle_sibling()
has its main weakness here.No doubt that this patch could improve
performance over the current logic,but will still retain the major
drawback of searching for an idle cpu in a large socket in the worst case.
> 
> It should has some help on burst wake up benchmarks like aim7.
> 
> Original-patch-by: Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxx>
> ---
>  kernel/sched/fair.c |   40 +++++++++++++++++++---------------------
>  1 files changed, 19 insertions(+), 21 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e116215..fa40e49 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3253,13 +3253,13 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
>  /*
>   * Try and locate an idle CPU in the sched_domain.
>   */
> -static int select_idle_sibling(struct task_struct *p)
> +static int select_idle_sibling(struct task_struct *p,
> +				struct sched_domain *affine_sd, int sync)

As Namhyung pointed out where is affine_sd being used?
>  {
>  	int cpu = smp_processor_id();
>  	int prev_cpu = task_cpu(p);
>  	struct sched_domain *sd;
>  	struct sched_group *sg;
> -	int i;
> 
>  	/*
>  	 * If the task is going to be woken-up on this cpu and if it is
> @@ -3281,27 +3281,25 @@ static int select_idle_sibling(struct task_struct *p)
>  	/*
>  	 * Otherwise, iterate the domains and find an elegible idle cpu.
>  	 */
> -	sd = rcu_dereference(per_cpu(sd_llc, prev_cpu));
> -	for_each_lower_domain(sd) {
> +	for_each_domain(prev_cpu, sd) {

Why is prev_cpu being used? Ideally it should be the target cpu
(waking/prev) depending on what wake_affine() decides.

>  		sg = sd->groups;
>  		do {
> -			if (!cpumask_intersects(sched_group_cpus(sg),
> -						tsk_cpus_allowed(p)))
> -				goto next;
> -
> -			for_each_cpu(i, sched_group_cpus(sg)) {
> -				if (!idle_cpu(i))
> -					goto next;
> -			}
> -
> -			prev_cpu = cpumask_first_and(sched_group_cpus(sg),
> -					tsk_cpus_allowed(p));
> -			goto done;
> -next:
> -			sg = sg->next;
> -		} while (sg != sd->groups);
> +			int nr_busy = atomic_read(&sg->sgp->nr_busy_cpus);
> +			int i;
> +
> +			/* no idle cpu in the group */
> +			if (nr_busy == sg->group_weight)
> +				continue;
> +			for_each_cpu_and(i, sched_group_cpus(sg),
> +							tsk_cpus_allowed(p))
> +				if (idle_cpu(i))
> +					return i;
> +		} while (sg = sg->next, sg != sd->groups);
> +
> +		/* only wake up task on the same cpu socket as prev cpu */
> +		if (sd == per_cpu(sd_llc, prev_cpu))
> +			break;
>  	}
> -done:
>  	return prev_cpu;

target cpu needs to be returned right? I dont understand why prev_cpu is
considered everywhere.

>  }
> 
> @@ -3355,7 +3353,7 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
>  	}
> 
>  	if (affine_sd) {
> -		new_cpu = select_idle_sibling(p, prev_cpu);
> +		new_cpu = select_idle_sibling(p, affine_sd, sync);
>  		goto unlock;
>  	}

To overcome the drawback of large sockets, I had suggested using
blocked_load+runnable_load of PJT's metric and in select_idle_sibling()
querying only the L2 cache domains.

https://lkml.org/lkml/2013/1/8/619

What do you think about this?


> 

Regards
Preeti U Murthy

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