Re: [PATCH v3 1/2] sched: smart wake-affine foundation

[Sam Ben <sam.bennn@gmail.com>]

On 07/08/2013 10:36 AM, Michael Wang wrote:
> Hi, Sam
>
> On 07/07/2013 09:31 AM, Sam Ben wrote:
>> On 07/04/2013 12:55 PM, Michael Wang wrote:
>>> wake-affine stuff is always trying to pull wakee close to waker, by
>>> theory,
>>> this will bring benefit if waker's cpu cached hot data for wakee, or the
>>> extreme ping-pong case.
>> What's the meaning of ping-pong case?
> PeterZ explained it well in here:
>
> 	https://lkml.org/lkml/2013/3/7/332
>
> And you could try to compare:
> 	taskset 1 perf bench sched pipe
> with
> 	perf bench sched pipe

Why sched pipe is special?

>
> to confirm it ;-)
>
> Regards,
> Michael Wang
>
>>> And testing show it could benefit hackbench 15% at most.
>>>
>>> However, the whole stuff is somewhat blindly and time-consuming, some
>>> workload therefore suffer.
>>>
>>> And testing show it could damage pgbench 50% at most.
>>>
>>> Thus, wake-affine stuff should be more smart, and realise when to stop
>>> it's thankless effort.
>>>
>>> This patch introduced 'nr_wakee_switch', which will be increased each
>>> time the task switch it's wakee.
>>>
>>> So a high 'nr_wakee_switch' means the task has more than one wakee, and
>>> bigger the number, higher the wakeup frequency.
>>>
>>> Now when making the decision on whether to pull or not, pay attention on
>>> the wakee with a high 'nr_wakee_switch', pull such task may benefit
>>> wakee,
>>> but also imply that waker will face cruel competition later, it could be
>>> very cruel or very fast depends on the story behind 'nr_wakee_switch',
>>> whatever, waker therefore suffer.
>>>
>>> Furthermore, if waker also has a high 'nr_wakee_switch', imply that
>>> multiple
>>> tasks rely on it, then waker's higher latency will damage all of them,
>>> pull
>>> wakee seems to be a bad deal.
>>>
>>> Thus, when 'waker->nr_wakee_switch / wakee->nr_wakee_switch' become
>>> higher
>>> and higher, the deal seems to be worse and worse.
>>>
>>> The patch therefore help wake-affine stuff to stop it's work when:
>>>
>>>      wakee->nr_wakee_switch > factor &&
>>>      waker->nr_wakee_switch > (factor * wakee->nr_wakee_switch)
>>>
>>> The factor here is the node-size of current-cpu, so bigger node will lead
>>> to more pull since the trial become more severe.
>>>
>>> After applied the patch, pgbench show 40% improvement at most.
>>>
>>> Test:
>>>      Tested with 12 cpu X86 server and tip 3.10.0-rc7.
>>>
>>>      pgbench            base    smart
>>>
>>>      | db_size | clients |  tps  |    |  tps  |
>>>      +---------+---------+-------+   +-------+
>>>      | 22 MB   |       1 | 10598 |   | 10796 |
>>>      | 22 MB   |       2 | 21257 |   | 21336 |
>>>      | 22 MB   |       4 | 41386 |   | 41622 |
>>>      | 22 MB   |       8 | 51253 |   | 57932 |
>>>      | 22 MB   |      12 | 48570 |   | 54000 |
>>>      | 22 MB   |      16 | 46748 |   | 55982 | +19.75%
>>>      | 22 MB   |      24 | 44346 |   | 55847 | +25.93%
>>>      | 22 MB   |      32 | 43460 |   | 54614 | +25.66%
>>>      | 7484 MB |       1 |  8951 |   |  9193 |
>>>      | 7484 MB |       2 | 19233 |   | 19240 |
>>>      | 7484 MB |       4 | 37239 |   | 37302 |
>>>      | 7484 MB |       8 | 46087 |   | 50018 |
>>>      | 7484 MB |      12 | 42054 |   | 48763 |
>>>      | 7484 MB |      16 | 40765 |   | 51633 | +26.66%
>>>      | 7484 MB |      24 | 37651 |   | 52377 | +39.11%
>>>      | 7484 MB |      32 | 37056 |   | 51108 | +37.92%
>>>      | 15 GB   |       1 |  8845 |   |  9104 |
>>>      | 15 GB   |       2 | 19094 |   | 19162 |
>>>      | 15 GB   |       4 | 36979 |   | 36983 |
>>>      | 15 GB   |       8 | 46087 |   | 49977 |
>>>      | 15 GB   |      12 | 41901 |   | 48591 |
>>>      | 15 GB   |      16 | 40147 |   | 50651 | +26.16%
>>>      | 15 GB   |      24 | 37250 |   | 52365 | +40.58%
>>>      | 15 GB   |      32 | 36470 |   | 50015 | +37.14%
>>>
>>> CC: Ingo Molnar <mingo@kernel.org>
>>> CC: Peter Zijlstra <peterz@infradead.org>
>>> CC: Mike Galbraith <efault@gmx.de>
>>> Signed-off-by: Michael Wang <wangyun@linux.vnet.ibm.com>
>>> ---
>>>    include/linux/sched.h |    3 +++
>>>    kernel/sched/fair.c   |   47
>>> +++++++++++++++++++++++++++++++++++++++++++++++
>>>    2 files changed, 50 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>>> index 178a8d9..1c996c7 100644
>>> --- a/include/linux/sched.h
>>> +++ b/include/linux/sched.h
>>> @@ -1041,6 +1041,9 @@ struct task_struct {
>>>    #ifdef CONFIG_SMP
>>>        struct llist_node wake_entry;
>>>        int on_cpu;
>>> +    struct task_struct *last_wakee;
>>> +    unsigned long nr_wakee_switch;
>>> +    unsigned long last_switch_decay;
>>>    #endif
>>>        int on_rq;
>>>    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index c61a614..a4ddbf5 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -2971,6 +2971,23 @@ static unsigned long cpu_avg_load_per_task(int
>>> cpu)
>>>        return 0;
>>>    }
>>>    +static void record_wakee(struct task_struct *p)
>>> +{
>>> +    /*
>>> +     * Rough decay(wiping) for cost saving, don't worry
>>> +     * about the boundary, really active task won't care
>>> +     * the loose.
>>> +     */
>>> +    if (jiffies > current->last_switch_decay + HZ) {
>>> +        current->nr_wakee_switch = 0;
>>> +        current->last_switch_decay = jiffies;
>>> +    }
>>> +
>>> +    if (current->last_wakee != p) {
>>> +        current->last_wakee = p;
>>> +        current->nr_wakee_switch++;
>>> +    }
>>> +}
>>>      static void task_waking_fair(struct task_struct *p)
>>>    {
>>> @@ -2991,6 +3008,7 @@ static void task_waking_fair(struct task_struct *p)
>>>    #endif
>>>          se->vruntime -= min_vruntime;
>>> +    record_wakee(p);
>>>    }
>>>      #ifdef CONFIG_FAIR_GROUP_SCHED
>>> @@ -3109,6 +3127,28 @@ static inline unsigned long
>>> effective_load(struct task_group *tg, int cpu,
>>>      #endif
>>>    +static int wake_wide(struct task_struct *p)
>>> +{
>>> +    int factor = nr_cpus_node(cpu_to_node(smp_processor_id()));
>>> +
>>> +    /*
>>> +     * Yeah, it's the switching-frequency, could means many wakee or
>>> +     * rapidly switch, use factor here will just help to automatically
>>> +     * adjust the loose-degree, so bigger node will lead to more pull.
>>> +     */
>>> +    if (p->nr_wakee_switch > factor) {
>>> +        /*
>>> +         * wakee is somewhat hot, it needs certain amount of cpu
>>> +         * resource, so if waker is far more hot, prefer to leave
>>> +         * it alone.
>>> +         */
>>> +        if (current->nr_wakee_switch > (factor * p->nr_wakee_switch))
>>> +            return 1;
>>> +    }
>>> +
>>> +    return 0;
>>> +}
>>> +
>>>    static int wake_affine(struct sched_domain *sd, struct task_struct
>>> *p, int sync)
>>>    {
>>>        s64 this_load, load;
>>> @@ -3118,6 +3158,13 @@ static int wake_affine(struct sched_domain *sd,
>>> struct task_struct *p, int sync)
>>>        unsigned long weight;
>>>        int balanced;
>>>    +    /*
>>> +     * If we wake multiple tasks be careful to not bounce
>>> +     * ourselves around too much.
>>> +     */
>>> +    if (wake_wide(p))
>>> +        return 0;
>>> +
>>>        idx      = sd->wake_idx;
>>>        this_cpu  = smp_processor_id();
>>>        prev_cpu  = task_cpu(p);
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