Messages in this thread | | | Subject | Re: [PATCH] sched/cfs: change initial value of runnable_avg | From | Holger Hoffstätte <> | Date | Thu, 25 Jun 2020 12:42:29 +0200 |
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On 2020-06-25 11:56, Vincent Guittot wrote: > On Thu, 25 Jun 2020 at 11:24, Holger Hoffstätte > <holger@applied-asynchrony.com> wrote: >> >> On 2020-06-24 17:44, Vincent Guittot wrote: >>> Some performance regression on reaim benchmark have been raised with >>> commit 070f5e860ee2 ("sched/fair: Take into account runnable_avg to classify group") >>> >>> The problem comes from the init value of runnable_avg which is initialized >>> with max value. This can be a problem if the newly forked task is finally >>> a short task because the group of CPUs is wrongly set to overloaded and >>> tasks are pulled less agressively. >>> >>> Set initial value of runnable_avg equals to util_avg to reflect that there >>> is no waiting time so far. >>> >>> Fixes: 070f5e860ee2 ("sched/fair: Take into account runnable_avg to classify group") >>> Reported-by: kernel test robot <rong.a.chen@intel.com> >>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> >>> --- >>> kernel/sched/fair.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>> index 0424a0af5f87..45e467bf42fc 100644 >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> @@ -806,7 +806,7 @@ void post_init_entity_util_avg(struct task_struct *p) >>> } >>> } >>> >>> - sa->runnable_avg = cpu_scale; >>> + sa->runnable_avg = sa->util_avg; >>> >>> if (p->sched_class != &fair_sched_class) { >>> /* >>> >> >> Something is wrong here. I woke up my machine from suspend-to-RAM this morning >> and saw that a completely idle machine had a loadavg of ~7. According to my > > Just to make sure: Are you speaking about loadavg that is output by > /proc/loadavg or load_avg which is the PELT load ?
/proc/loadavg
>> monitoring system this happened to be the loadavg right before I suspended. >> I've reverted this, rebooted, created a loadavg >0, suspended and after wake up >> loadavg again correctly ranges between 0 and whatever, as expected. > > I'm not sure to catch why ~7 is bad compared to correctly ranges > between 0 and whatever. Isn't ~7 part of the whatever ?
After wakeup the _baseline_ for loadavg seemed to be the last value before suspend, not 0. The 7 then was the base loadavg for a _mostly idle machine_ (just reading mail etc.), i.e. it never went below said baseline again, no matter the _actual_ load.
Here's an image: https://imgur.com/a/kd2stqO
Before 02:00 last night the load was ~7 (compiled something), then all processes were terminated and the machine was suspended. After wakeup the machine was mostly idle (9am..11am), yet measured loadavg continued with the same value as before. I didn't notice this right away since my CPU meter on the desktop didn't show any *actual* activity (because there was none). The spike at ~11am is the revert/reboot. After that loadavg became normal again, i.e. representative of the actual load, even after suspend/resume cycles. I suspend/resume every night and the only thing that changed recently was this patch, so.. :)
-h
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