Messages in this thread Patch in this message | | | Subject | Re: [PATCH 4/4] sched: Allow tasks to stack with a workqueue on the same CPU | From | Mike Galbraith <> | Date | Mon, 18 Dec 2017 11:44:30 +0100 |
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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@techsingularity.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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