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

[Michael Wang]

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

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@xxxxxxxxxx>
>> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> CC: Mike Galbraith <efault@xxxxxx>
>> Signed-off-by: Michael Wang <wangyun@xxxxxxxxxxxxxxxxxx>
>> ---
>>   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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