From: Vincent Guittot <vincent.guittot@linaro.org>
To: Tobias Huschle <huschle@linux.ibm.com>
Cc: Luis Machado <luis.machado@arm.com>,
linux-kernel@vger.kernel.org, mingo@redhat.com,
peterz@infradead.org, juri.lelli@redhat.com,
dietmar.eggemann@arm.com, rostedt@goodmis.org,
bsegall@google.com, mgorman@suse.de, bristot@redhat.com,
vschneid@redhat.com, sshegde@linux.vnet.ibm.com,
srikar@linux.vnet.ibm.com, linuxppc-dev@lists.ozlabs.org,
nd <nd@arm.com>
Subject: Re: [RFC] sched/eevdf: sched feature to dismiss lag on wakeup
Date: Tue, 19 Mar 2024 14:41:14 +0100 [thread overview]
Message-ID: <CAKfTPtDyrsnq-CSFo+upzdOJpuH=JkRzSALad-OL29OvqkK2dg@mail.gmail.com> (raw)
In-Reply-To: <4b25ab45b762e64b9df09d4d12d8289f@linux.ibm.com>
On Tue, 19 Mar 2024 at 10:08, Tobias Huschle <huschle@linux.ibm.com> wrote:
>
> On 2024-03-18 15:45, Luis Machado wrote:
> > On 3/14/24 13:45, Tobias Huschle wrote:
> >> On Fri, Mar 08, 2024 at 03:11:38PM +0000, Luis Machado wrote:
> >>> On 2/28/24 16:10, Tobias Huschle wrote:
> >>>>
> >>>> Questions:
> >>>> 1. The kworker getting its negative lag occurs in the following
> >>>> scenario
> >>>> - kworker and a cgroup are supposed to execute on the same CPU
> >>>> - one task within the cgroup is executing and wakes up the
> >>>> kworker
> >>>> - kworker with 0 lag, gets picked immediately and finishes its
> >>>> execution within ~5000ns
> >>>> - on dequeue, kworker gets assigned a negative lag
> >>>> Is this expected behavior? With this short execution time, I
> >>>> would
> >>>> expect the kworker to be fine.
> >>>
> >>> That strikes me as a bit odd as well. Have you been able to determine
> >>> how a negative lag
> >>> is assigned to the kworker after such a short runtime?
> >>>
> >>
> >> I did some more trace reading though and found something.
> >>
> >> What I observed if everything runs regularly:
> >> - vhost and kworker run alternating on the same CPU
> >> - if the kworker is done, it leaves the runqueue
> >> - vhost wakes up the kworker if it needs it
> >> --> this means:
> >> - vhost starts alone on an otherwise empty runqueue
> >> - it seems like it never gets dequeued
> >> (unless another unrelated task joins or migration hits)
> >> - if vhost wakes up the kworker, the kworker gets selected
> >> - vhost runtime > kworker runtime
> >> --> kworker gets positive lag and gets selected immediately next
> >> time
> >>
> >> What happens if it does go wrong:
> >> From what I gather, there seem to be occasions where the vhost either
> >> executes suprisingly quick, or the kworker surprinsingly slow. If
> >> these
> >> outliers reach critical values, it can happen, that
> >> vhost runtime < kworker runtime
> >> which now causes the kworker to get the negative lag.
> >>
> >> In this case it seems like, that the vhost is very fast in waking up
> >> the kworker. And coincidentally, the kworker takes, more time than
> >> usual
> >> to finish. We speak of 4-digit to low 5-digit nanoseconds.
> >>
> >> So, for these outliers, the scheduler extrapolates that the kworker
> >> out-consumes the vhost and should be slowed down, although in the
> >> majority
> >> of other cases this does not happen.
> >
> > Thanks for providing the above details Tobias. It does seem like EEVDF
> > is strict
> > about the eligibility checks and making tasks wait when their lags are
> > negative, even
> > if just a little bit as in the case of the kworker.
> >
> > There was a patch to disable the eligibility checks
> > (https://lore.kernel.org/lkml/20231013030213.2472697-1-youssefesmat@chromium.org/),
> > which would make EEVDF more like EVDF, though the deadline comparison
> > would
> > probably still favor the vhost task instead of the kworker with the
> > negative lag.
> >
> > I'm not sure if you tried it, but I thought I'd mention it.
>
> Haven't seen that one yet. Unfortunately, it does not help to ignore the
> eligibility.
>
> I'm inclined to rather propose propose a documentation change, which
> describes that tasks should not rely on woken up tasks being scheduled
> immediately.
Where do you see such an assumption ? Even before eevdf, there were
nothing that ensures such behavior. When using CFS (legacy or eevdf)
tasks, you can't know if the newly wakeup task will run 1st or not
>
> Changing things in the code to address for the specific scenario I'm
> seeing seems to mostly create unwanted side effects and/or would require
> the definition of some magic cut-off values.
>
>
next prev parent reply other threads:[~2024-03-19 13:41 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-28 16:10 [RFC] sched/eevdf: sched feature to dismiss lag on wakeup Tobias Huschle
2024-02-29 3:36 ` K Prateek Nayak
2024-03-06 11:31 ` Tobias Huschle
2024-03-08 15:11 ` Luis Machado
2024-03-14 13:45 ` Tobias Huschle
2024-03-18 14:45 ` Luis Machado
2024-03-19 9:08 ` Tobias Huschle
2024-03-19 13:41 ` Vincent Guittot [this message]
2024-03-20 7:04 ` Tobias Huschle
2024-03-20 8:12 ` Luis Machado
[not found] ` <65fa8a7c.050a0220.c8ec5.0278SMTPIN_ADDED_BROKEN@mx.google.com>
2024-03-20 13:51 ` Vincent Guittot
2024-03-21 12:18 ` Tobias Huschle
[not found] ` <65fc25ae.810a0220.f705f.4cdbSMTPIN_ADDED_BROKEN@mx.google.com>
2024-03-22 17:02 ` Vincent Guittot
2024-04-09 7:35 ` Tobias Huschle
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CAKfTPtDyrsnq-CSFo+upzdOJpuH=JkRzSALad-OL29OvqkK2dg@mail.gmail.com' \
--to=vincent.guittot@linaro.org \
--cc=bristot@redhat.com \
--cc=bsegall@google.com \
--cc=dietmar.eggemann@arm.com \
--cc=huschle@linux.ibm.com \
--cc=juri.lelli@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linuxppc-dev@lists.ozlabs.org \
--cc=luis.machado@arm.com \
--cc=mgorman@suse.de \
--cc=mingo@redhat.com \
--cc=nd@arm.com \
--cc=peterz@infradead.org \
--cc=rostedt@goodmis.org \
--cc=srikar@linux.vnet.ibm.com \
--cc=sshegde@linux.vnet.ibm.com \
--cc=vschneid@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).