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

>
> 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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