Messages in this thread | | | From | Doug Anderson <> | Date | Wed, 24 Jun 2020 11:29:47 -0700 | Subject | Re: [PATCH] cros_ec_spi: Even though we're RT priority, don't bump cpu freq |
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Hi,
On Wed, Jun 24, 2020 at 10:55 AM Joel Fernandes <joelaf@google.com> wrote: > > On Wed, Jun 24, 2020 at 1:52 PM Qais Yousef <qais.yousef@arm.com> wrote: > > > > On 06/24/20 13:35, Joel Fernandes wrote: > > > > [...] > > > > > > Doing the in-kernel opt-out via API should be fine, I think. But this will > > > > need to be discussed in the wider circle. It will already clash with this for > > > > example > > > > > > > > https://lore.kernel.org/lkml/20200619172011.5810-1-qais.yousef@arm.com/ > > > > > > Have not yet looked closer at that patch, but are you saying this > > > patch clashes with that work? Sorry I am operating on 2 hours of sleep > > > here. > > > > The series is an optimization to remove the uclamp overhead from the scheduler > > fastpath until the userspace uses it. It introduces a static key that is > > disabled by default and will cause uclamp logic not to execute in the fast > > path. Once the userspace starts using util clamp, which we detect by either > > > > 1. Changing uclamp value of a task with sched_setattr() > > 2. Modifying the default sysctl_sched_util_clamp_{min, max} > > 3. Modifying the default cpu.uclamp.{min, max} value in cgroup > > > > If we start having in-kernel users changing uclamp value this means drivers > > will cause the system to opt-in into uclamp automatically even if the > > userspace doesn't actually use it. > > > > I think we can solve this by providing a special API to opt-out safely. Which > > is the right thing to do anyway even if we didn't have this clash. > > Makes sense, thanks.
OK, so I think the summary is:
1. There are enough external dependencies that are currently in the works that it makes sense for those to land first without trying to cram my patch to cros_ec in.
2. Maybe, as part of the work that's already going on, someone will add an API that I can use. If so then I can write my patch once that lands.
3. If nobody adds an API then I could look at adding the API myself once everything else is settled.
4. It looks as if the patch you mentioned originally [1] that allows userspace to just fully disable uclamp for RT tasks will land eventually (if we're stuck for a short term solution we can pick the existing patch). I believe Chrome OS will use that to just fully disable the default boosting for RT tasks across our system and (if needed) add boosts on a case-by-case basis. Once we do that, the incentive to land a patch for cros_ec will be mostly gone and probably we could consider my patch abandoned. While it would technically still be sane to land it won't have any real-world benefit.
[1] https://lore.kernel.org/r/20200511154053.7822-1-qais.yousef@arm.com
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