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Date: Mon, 03 Jun 2013 10:28:58 +0800
From: Michael Wang <wangyun@...ux.vnet.ibm.com>
To: LKML <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <peterz@...radead.org>
CC: 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: [RFC PATCH] sched: smart wake-affine
On 05/28/2013 01:05 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.
>
> 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 smarter, and realise when to stop
> it's thankless effort.
Is there any comments?
Peter, do you have any comments on this idea? Is this the kind of fix we
are looking for? I think you mentioned we want some kind of filter
rather than the knob, correct?
Folks, please let me know your concerns so I could help on the research
work :)
Regards,
Michael Wang
>
> This patch introduced per task '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
> less the wakee 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 that imply waker will face cruel competition later, it could be very
> crule 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', that imply multiple
> tasks rely on it, waker's higher latency will damage all those tasks, pull
> wakee in such cases 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.
>
> This 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 online cpu number, so more cpu will lead to more pull
> since the trial become more severe.
>
> After applied the patch, pgbench show 42% improvement at most.
>
> Test:
> Test with 12 cpu X86 server and tip 3.10.0-rc1.
>
> base smart
>
> | db_size | clients | tps | | tps |
> +---------+---------+-------+ +-------+
> | 21 MB | 1 | 10749 | | 10337 |
> | 21 MB | 2 | 21382 | | 21391 |
> | 21 MB | 4 | 41570 | | 41808 |
> | 21 MB | 8 | 52828 | | 58792 |
> | 21 MB | 12 | 48447 | | 54553 |
> | 21 MB | 16 | 46246 | | 56726 | +22.66%
> | 21 MB | 24 | 43850 | | 56853 | +29.65%
> | 21 MB | 32 | 43455 | | 55846 | +28.51%
> | 7483 MB | 1 | 9290 | | 8848 |
> | 7483 MB | 2 | 19347 | | 19351 |
> | 7483 MB | 4 | 37135 | | 37511 |
> | 7483 MB | 8 | 47310 | | 50210 |
> | 7483 MB | 12 | 42721 | | 49396 |
> | 7483 MB | 16 | 41016 | | 51826 | +26.36%
> | 7483 MB | 24 | 37540 | | 52579 | +40.06%
> | 7483 MB | 32 | 36756 | | 51332 | +39.66%
> | 15 GB | 1 | 8758 | | 8670 |
> | 15 GB | 2 | 19204 | | 19249 |
> | 15 GB | 4 | 36997 | | 37199 |
> | 15 GB | 8 | 46578 | | 50681 |
> | 15 GB | 12 | 42141 | | 48671 |
> | 15 GB | 16 | 40518 | | 51280 | +26.56%
> | 15 GB | 24 | 36788 | | 52329 | +42.24%
> | 15 GB | 32 | 36056 | | 50350 | +39.64%
>
>
>
> 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 | 45 +++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 48 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 f62b16d..eaaceb7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3127,6 +3127,45 @@ static inline unsigned long effective_load(struct task_group *tg, int cpu,
>
> #endif
>
> +static void record_wakee(struct task_struct *p)
> +{
> + /*
> + * Rough decay, 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 int nasty_pull(struct task_struct *p)
> +{
> + int factor = cpumask_weight(cpu_online_mask);
> +
> + /*
> + * 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 more cpu 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;
> @@ -3136,6 +3175,9 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
> unsigned long weight;
> int balanced;
>
> + if (nasty_pull(p))
> + return 0;
> +
> idx = sd->wake_idx;
> this_cpu = smp_processor_id();
> prev_cpu = task_cpu(p);
> @@ -3428,6 +3470,9 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
> /* while loop will break here if sd == NULL */
> }
> unlock:
> + if (sd_flag & SD_BALANCE_WAKE)
> + record_wakee(p);
> +
> rcu_read_unlock();
>
> return new_cpu;
>
--
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