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Message-ID: <1513593870.6876.27.camel@gmx.de>
Date: Mon, 18 Dec 2017 11:44:30 +0100
From: Mike Galbraith <efault@....de>
To: Mel Gorman <mgorman@...hsingularity.net>,
Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>,
Matt Fleming <matt@...eblueprint.co.uk>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] sched: Allow tasks to stack with a workqueue on the
same CPU
On Mon, 2017-12-18 at 09:43 +0000, Mel Gorman wrote:
> If tasks wake a kworker to do some work and is woken on completion and it
> was a per-cpu kworker that was used then a situation can arise where the
> current CPU is always active when the kworker is waking and select_idle_sibling
> moves the task. This leads to a situation where a task moves around the socket
> each time a kworker is used even through the relationship is effectively sync.
> This patch special cases a kworker running on the same CPU. It has a noticable
> impact on migrations and performance of dbench running with the XFS filesystem
> but has no impact on ext4 as ext4 interacts with a kthread, not a kworker.
I think intentional stacking is a very bad idea unless you know with
absolute certainty that waker/wakee are in fact 100% synchronous. This
is IMO the wrong way to go about combating the excessive bouncing, that
can be achieved by simple ratelimiting.
> 4.15.0-rc3 4.15.0-rc3
> wakeprev stackwq
> Hmean 1 392.92 ( 0.00%) 1024.22 ( 160.67%)
> Hmean 2 787.09 ( 0.00%) 1808.38 ( 129.75%)
> Hmean 4 1559.71 ( 0.00%) 2525.42 ( 61.92%)
> Hmean 8 2576.05 ( 0.00%) 2881.12 ( 11.84%)
> Hmean 16 2949.28 ( 0.00%) 3137.65 ( 6.39%)
> Hmean 32 3041.89 ( 0.00%) 3147.92 ( 3.49%)
> Hmean 64 1655.42 ( 0.00%) 1756.21 ( 6.09%)
> Hmean 128 1133.19 ( 0.00%) 1165.39 ( 2.84%)
> Stddev 1 2.59 ( 0.00%) 11.21 (-332.82%)
> Stddev 2 8.96 ( 0.00%) 13.57 ( -51.44%)
> Stddev 4 20.15 ( 0.00%) 8.51 ( 57.75%)
> Stddev 8 17.15 ( 0.00%) 14.45 ( 15.75%)
> Stddev 16 30.29 ( 0.00%) 31.30 ( -3.33%)
> Stddev 32 64.45 ( 0.00%) 57.22 ( 11.21%)
> Stddev 64 55.89 ( 0.00%) 62.84 ( -12.43%)
> Stddev 128 55.89 ( 0.00%) 62.75 ( -12.27%)
>
> There is also a large drop in system CPU usage;
>
> 4.15.0-rc3 4.15.0-rc3
> wakeprev stackwq
> User 1561.85 1166.59
> System 6961.89 4965.09
> Elapsed 1472.05 1471.84
>
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> ---
> kernel/sched/fair.c | 29 +++++++++++++++++++++++++++--
> kernel/sched/features.h | 8 ++++++++
> 2 files changed, 35 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 95b1145bc38d..cff55481bd19 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5684,6 +5684,19 @@ static int wake_wide(struct task_struct *p)
> return 1;
> }
>
> +/*
> + * Returns true if a wakeup is either from or to a workqueue and the tasks
> + * appear to be synchronised with each other.
> + */
> +static bool
> +is_wakeup_workqueue_sync(struct task_struct *p, int this_cpu, int prev_cpu)
> +{
> + return sched_feat(WA_STACK_WQ) &&
> + this_cpu == prev_cpu &&
> + ((p->flags & PF_WQ_WORKER) || (current->flags & PF_WQ_WORKER)) &&
> + this_rq()->nr_running <= 1;
> +}
> +
> /*
> * The purpose of wake_affine() is to quickly determine on which CPU we can run
> * soonest. For the purpose of speed we only consider the waking and previous
> @@ -5735,7 +5748,7 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync)
> }
>
> static int
> -wake_affine_sync(int this_cpu, int sync)
> +wake_affine_sync(struct task_struct *p, int this_cpu, int prev_cpu, int sync)
> {
> /*
> * Consider stacking tasks if it's a sync wakeup and there is only
> @@ -5745,6 +5758,14 @@ wake_affine_sync(int this_cpu, int sync)
> if (sync && cpu_rq(this_cpu)->nr_running == 1)
> return this_cpu;
>
> + /*
> + * If the waker or wakee is a workqueue and it appears to be similar
> + * to a sync wakeup then assume the waker will sleep shortly and allow
> + * the tasks to stack on the same CPU.
> + */
> + if (is_wakeup_workqueue_sync(p, this_cpu, prev_cpu))
> + return this_cpu;
> +
> return nr_cpumask_bits;
> }
>
> @@ -5794,7 +5815,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
> new_cpu = wake_affine_idle(this_cpu, prev_cpu, sync);
>
> if (sched_feat(WA_IDLE) && new_cpu == nr_cpumask_bits)
> - new_cpu = wake_affine_sync(this_cpu, sync);
> + new_cpu = wake_affine_sync(p, this_cpu, prev_cpu, sync);
>
> if (sched_feat(WA_WEIGHT) && new_cpu == nr_cpumask_bits)
> new_cpu = wake_affine_weight(sd, p, this_cpu, prev_cpu, sync);
> @@ -6240,6 +6261,10 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> if (idle_cpu(target))
> return target;
>
> + /* Allow a wakeup to stack if it looks like a synchronous workqueue */
> + if (is_wakeup_workqueue_sync(p, smp_processor_id(), target))
> + return target;
> +
> /*
> * If the previous cpu is cache affine and idle, don't be stupid.
> */
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index 9552fd5854bf..c96ad246584a 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -85,3 +85,11 @@ SCHED_FEAT(ATTACH_AGE_LOAD, true)
> SCHED_FEAT(WA_IDLE, true)
> SCHED_FEAT(WA_WEIGHT, true)
> SCHED_FEAT(WA_BIAS, true)
> +
> +/*
> + * If true then a process may stack with a workqueue on the same CPU during
> + * wakeup instead of finding an idle sibling. This should only happen in the
> + * case where there appears to be a strong relationship beween the wq and the
> + * task e.g. IO operations dispatched to a workqueue on XFS.
> + */
> +SCHED_FEAT(WA_STACK_WQ, true)
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