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Message-ID: <51DA25A4.2050803@linux.vnet.ibm.com>
Date: Mon, 08 Jul 2013 10:36:20 +0800
From: Michael Wang <wangyun@...ux.vnet.ibm.com>
To: Sam Ben <sam.bennn@...il.com>
CC: LKML <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Mike Galbraith <efault@....de>, Alex Shi <alex.shi@...el.com>,
Namhyung Kim <namhyung@...nel.org>,
Paul Turner <pjt@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
Ram Pai <linuxram@...ibm.com>
Subject: Re: [PATCH v3 1/2] sched: smart wake-affine foundation
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@...nel.org>
>> CC: Peter Zijlstra <peterz@...radead.org>
>> CC: Mike Galbraith <efault@....de>
>> Signed-off-by: Michael Wang <wangyun@...ux.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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