Messages in this thread | | | Date | Tue, 14 Apr 2020 12:29:01 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH 4/4] sched/deadline: Implement fallback mechanism for !fit case |
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On 04/09/20 20:43, Dietmar Eggemann wrote: > On 09.04.20 16:55, Qais Yousef wrote: > > Hi Luca > > > > On 04/09/20 15:00, luca abeni wrote: > > [...] > > >> There is already a tunable for the SCHED_DEADLINE admission test > >> (/proc/sys/kernel/sched_rt_{runtime,period}_us, if I understand well > >> what you are suggesting). The problem is that it is not easy to find a > >> value for this tunable that guarantees the hard respect of all > >> deadlines. > > > > I don't think it's similar to what I was referring to. But my knowledge about > > DL could have failed me to fully appreciate what you're saying. > > > > This tunable for RT prevents a single task from using 100% CPU time. I think > > DL uses it in a similar manner. > > > > What I meant by overcommiting, is allowing more DL tasks than the system can > > guarantee to meet their deadlines. > > > > For example, in the context of capacity awareness, if you have 2 big cores, but > > 4 DL tasks request a bandwidth that can only be satisfied by the big cores, > > then 2 of them will miss their deadlines (almost) consistently, IIUC. > > __dl_overflow() now uses > > X = cap_scale(dl_b->bw, rd_capacity(cpu)) instead of X = cpus > > in > > dl_b->bw * X < dl_b->total_bw - old_bw + new_bw; > > > As an example, Arm64 Hikey64 with 4 LITTLE and 4 big CPUs uses now > (4x462 + 4x1024)/1024 = 5944/1024 ~ 5,80 instead of 8 CPUs. > > On mainline, the rt-app tests: > > "tasks" : { > "thread0" : { > "policy" : "SCHED_DEADLINE", > "instance" : 8, > "timer" : {"ref" : "unique0", "period" : 16000, "mode" : "absolute" }, > "run" : 10000, > "dl-runtime" :11000, > "dl-period" : 16000, > "dl-deadline" : 16000 > } > } > > is admitted on this board whereas with the patchset some of the tasks > are rejected: > > [rt-app] <crit> [7] sched_setattr returned -1 > sched_setattr: failed to set deadline attributes: Device or resource busy > > --- > > IMHO, one of the 3 places where DL Admission Control hooks into is: > > __sched_setscheduler -> sched_dl_overflow() ->__dl_overflow() > > [...]
Thanks Dietmar! I should have looked at the code with more intent.
-- Qais Yousef
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