Messages in this thread | | | Date | Tue, 29 Oct 2019 14:02:19 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] sched: rt: Make RT capacity aware |
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On Tue, Oct 29, 2019 at 01:54:24PM +0100, Vincent Guittot wrote: > On Tue, 29 Oct 2019 at 13:46, Qais Yousef <qais.yousef@arm.com> wrote: > > > > On 10/29/19 13:20, Vincent Guittot wrote: > > > > > Making big cores the default CPUs for all RT tasks is not a minor > > > > > change and IMO locality should stay the default behavior when there is > > > > > no uclamp constraint > > > > > > > > How this is affecting locality? The task will always go to the big core, so it > > > > should be local. > > > > > > local with the waker > > > You will force rt task to run on big cluster although waker, data and > > > interrupts can be on little one. > > > So making big core as default is far from always being the best choice > > > > This is loaded with assumptions IMO. AFAICT we don't know what's the best > > choice. > > > > First, the value of uclamp.min is outside of the scope of this patch. Unless > > what you're saying is that when uclamp.min is 1024 then we should NOT choose a > > big cpu then there's no disagreement about what this patch do. If that's what > > you're objecting to please be more specific about how do you see this working > > instead. > > My point is that this patch makes the big cores the default CPUs for > RT tasks which is far from being a minor change and far from being an > obvious default good choice
FIFO/RR tasks don't have a bandwidth specification (barring uclamp), therefore we must assume the worst. This is the same principle that has them select max_freq all the time.
I think it is a very natural extention of that very principle to place (otherwise unconstrained RT tasks) on big cores.
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