Messages in this thread | | | From | Joel Fernandes <> | Date | Fri, 4 Nov 2022 15:12:36 +0000 | Subject | Re: [PATCH v7 6/9] sched/fair: Add sched group latency support |
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On Fri, Nov 4, 2022 at 3:03 PM Vincent Guittot <vincent.guittot@linaro.org> wrote: [...] > > > > > > On 11/03/22 09:46, Vincent Guittot wrote: > > > > > > > On Tue, 1 Nov 2022 at 20:28, Qais Yousef <qyousef@layalina.io> wrote: > > > > > > > > > > > > > > > > On 10/28/22 11:34, Vincent Guittot wrote: > > > > > > > > > Task can set its latency priority with sched_setattr(), which is then used > > > > > > > > > to set the latency offset of its sched_enity, but sched group entities > > > > > > > > > still have the default latency offset value. > > > > > > > > > > > > > > > > > > Add a latency.nice field in cpu cgroup controller to set the latency > > > > > > > > > priority of the group similarly to sched_setattr(). The latency priority > > > > > > > > > is then used to set the offset of the sched_entities of the group. > > > > > > > > > > > > > > > > > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > > > > > > > > > --- > > > > > > > > > Documentation/admin-guide/cgroup-v2.rst | 8 ++++ > > > > > > > > > kernel/sched/core.c | 52 +++++++++++++++++++++++++ > > > > > > > > > kernel/sched/fair.c | 33 ++++++++++++++++ > > > > > > > > > kernel/sched/sched.h | 4 ++ > > > > > > > > > 4 files changed, 97 insertions(+) > > > > > > > > > > > > > > > > > > diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst > > > > > > > > > index be4a77baf784..d8ae7e411f9c 100644 > > > > > > > > > --- a/Documentation/admin-guide/cgroup-v2.rst > > > > > > > > > +++ b/Documentation/admin-guide/cgroup-v2.rst > > > > > > > > > @@ -1095,6 +1095,14 @@ All time durations are in microseconds. > > > > > > > > > values similar to the sched_setattr(2). This maximum utilization > > > > > > > > > value is used to clamp the task specific maximum utilization clamp. > > > > > > > > > > > > > > > > > > + cpu.latency.nice [...] > > > > > > > > > > > > > > Hmm so you are speaking about something that is not part of the patch. > > > > > > > Let focus on the patchset for now > > > > > > > > > > > > I am focusing on this patchset. Isn't this an essential part of the design? > > > > > > Once the interface is out we can't change it. As it stands, I can't see how it > > > > > > > > > > So, are you speaking about the interface i.e. setting a value between [-20:19] > > > > > > > > About how the cgroup and per task interface interact. > > > > > > > > How to get the effective value of latency_nice for a task that belongs to > > > > a hierarchy? > > > > > > At the common parents level of the 2 entities that you want to compare > > > and root level if there no other entity to compare with > > > > > > > > > > > If I have a task that has p->latency_nice = 20 but it belongs to a cgroup that > > > > has tg->cpu.latency.nice = -19 > > > > > > according to what i said above, latency_nice = 20 inside the group and > > > -19 when comparing at the parent level > > > > > > > > > > > And I want to use this interface in EAS; how should I interpret these values? > > > > How should I walk up the hierarchy and decide the _effective_ latency_nice > > > > value? > > > > > > The current use of latency_nice doesn't need to walk the hierarchy > > > because it applies at each scheduling level so the childs > > > automatically follow parents' latency. > > > > Not really, I don't see how that will work that way in the wake up path. The > > wake up path (EAS in particular) does not walk through CPU controller group > > hierarchy from top level, it only cares about cpuset/affinities and the > > "effective" values of tasks. > > I was explaining the current use of latency_ni i.e. in this patchset, > I'm not speaking about what should be done for other case like EAS
That's fine, but you did mention the negative value of latency_nice used to mark that a task prefers idle CPU so we should finish that discussion :-D. Since that will be one of the potential users of this patchset.
> In fact, it's exactly what I explained few lines above : > "> > At the common parents level of the 2 entities that you want to compare > > > and root level if there no other entity to compare with"
Yes this is a different usecase, but having more real world use cases will add more purpose to the patchset.
I also want to add -- for ChromeOS, Youssef tried it and the temporary boost that latency_nice gives is not enough for latency-sensitive workloads (like the main thread of a ChromeOS web page which is both CPU bound and handles latency-sensitive user input). So we are also exploring other ways. However, I have no issue with the patchset if it helps Android and would love to review further.
Thanks.
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