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....@intel.com>
> 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 <pre...@linux.vnet.ibm.com>
> Signed-off-by: Alex Shi <alex....@intel.com>
> ---
> 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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