Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 4 Nov 2022 11:37:03 +0100 | Subject | Re: [PATCH v7 6/9] sched/fair: Add sched group latency support |
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On Fri, 4 Nov 2022 at 11:15, Joel Fernandes <joel@joelfernandes.org> wrote: > > On Thu, Nov 3, 2022 at 5:03 PM Vincent Guittot > <vincent.guittot@linaro.org> wrote: > > > > On Thu, 3 Nov 2022 at 15:27, Qais Yousef <qyousef@layalina.io> 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 > > > > > > + A read-write single value file which exists on non-root > > > > > > + cgroups. The default is "0". > > > > > > + > > > > > > + The nice value is in the range [-20, 19]. > > > > > > + > > > > > > + This interface file allows reading and setting latency using the > > > > > > + same values used by sched_setattr(2). > > > > > > > > > > I'm still not sure about this [1]. > > > > > > > > I'm still not sure about what you are trying to say here ... > > > > > > > > This is about setting a latency nice prio to a group level. > > > > > > > > > > > > > > In some scenarios we'd like to get the effective latency_nice of the task. How > > > > > will the task inherit the cgroup value or be impacted by it? > > > > > > > > > > For example if there are tasks that belong to a latency sensitive cgroup; and > > > > > I'd like to skip some searches in EAS to improve that latency sensitivity - how > > > > > would I extract this info in EAS path given these tasks are using default > > > > > latency_nice value? And if should happen if their latency_nice is set to > > > > > something else other than default? > > > > > > > > > > [1] https://lore.kernel.org/lkml/20221012160734.hrkb5jcjdq7r23pr@wubuntu/ > > > > > > > > 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] > > > > > can be used to replace prefer_idle in cgroup as used in Android. I can't see > > > how this could happen if we don't define how the task will inherit the cgroup > > > value. If we can, mind elaborating how please? > > > > Or how to take into account the value set for a cgroup ? > > > > Regarding the behavior, the rule remains the same that a sched_entity > > attached to a cgroup will not get more (latency in this case) than > > what has been set for the group entity. > > I think the interface solves a different problem which is latency of > task or cgroup wrt other group. Vincent, you are setting this for a > “top app” group in android in your tests, and seeing improvement > correct? AFAICS, this improvement comes because of lower latency
Yes Top app and display group
> during *same CPU* competition between different groups by juggling > around the wakeup-preemption window -- which maybe is good for > Android. > > OTOH, the “prefer idle” flag in android that Qais is referring to, > will need a completely different method as I cannot see how a nice > value can communicate that (that can complement Vincent's changes > here). And it will need to have a per-task interface as well. We have
Why a negative latency_nice value condition can't be used ? or latency -20 ?
> something in ChromeOS as well, which is a proc knob and also > out-of-tree patch for that [1]. Without [1] we fail Android CTS > testing on a recent ARM64 ChromeOS device. > [1] https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3884575 > The changelog in [1] also has a detailed description of the ChromeOS usecase. > > Qais, any other reason you can see why Vincent's change will not be a > good thing for Android? Since you 1 CGroup for the whole user-facing > app (top app), you can just set that to a low "latency_nice" and get > better wake-up latency for that. > > (Side rant about latency and CFS -- IMHO a better long term solution > for lower latency is to use RT but don't throttle -- rather demote. Or > break CFS into multiple tiers, and apply demotion. This is in a way > what Vincent is doing, as the task becomes more CPU bound'ish, he's > taking away the latency boost. Vincent/Qais, somebody was working on > the RT demotion vs throttling a while back, any idea on the latest on > that?). > > thanks, > > - Joel > > > > > > > > > > > > > Thanks > > > > > > -- > > > Qais Yousef
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