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definitions=main-2311070195 X-Proofpoint-GUID: 5TILQTI7tXOgJwTPMpxVTE_ERzQ5kL38 X-Proofpoint-ORIG-GUID: 5TILQTI7tXOgJwTPMpxVTE_ERzQ5kL38 X-Rspamd-Queue-Id: 0D894C000D X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: 94x417woqh7wifxmo9819cm99e9swh4j X-HE-Tag: 1699400653-966269 X-HE-Meta: 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 GjXB/ubU xNtrnWgg3VHYXpgHnf0fLuCRBgnUbCw8WIMzHMpBI360vvnHIUyvrqv3VbTJ463HOftoUP+WmamU8oS8FKrhyxti0F8Piabk02bFT3bqWtWhqAcCSbO8SenOV1JyQQjjWRK0Z23W9IQLuNH55cAkLUq2BgFAylyH1If17XrPqInwHtavwrhjS8vtMPxxmTk/qerYeqS8hs+kdIZrNpErCOptfQNQGxjVjoozx/Uuhuoi2Iat+aDDvPFdgzl8xEZSLGuZ9USLqXjWaoY9gc/qDlneTejj75C+QplVpTfw+6Q6ANrhHUPWeWEYXPV/wLcZILdNeprsBuckfDRO73Ep7coFHhn1B+KDvmecDfsOn4d2TLmbO0Y058B0ajJbjEljR1aKoRZ2o6A04g/M/exk/hg8F5XLvrNY0E+D2C+JdlLxw42FGLWv0nNL5WA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Steven Rostedt writes: > On Tue, 7 Nov 2023 13:56:46 -0800 > Ankur Arora wrote: > >> Hi, > > Hi Ankur, > > Thanks for doing this! > >> >> We have two models of preemption: voluntary and full (and RT which is >> a fuller form of full preemption.) In this series -- which is based >> on Thomas' PoC (see [1]), we try to unify the two by letting the >> scheduler enforce policy for the voluntary preemption models as well. > > I would say there's "NONE" which is really just a "voluntary" but with > fewer preemption points ;-) But still should be mentioned, otherwise people > may get confused. > >> >> (Note that this is about preemption when executing in the kernel. >> Userspace is always preemptible.) >> > > >> Design >> == >> >> As Thomas outlines in [1], to unify the preemption models we >> want to: always have the preempt_count enabled and allow the scheduler >> to drive preemption policy based on the model in effect. >> >> Policies: >> >> - preemption=none: run to completion >> - preemption=voluntary: run to completion, unless a task of higher >> sched-class awaits >> - preemption=full: optimized for low-latency. Preempt whenever a higher >> priority task awaits. >> >> To do this add a new flag, TIF_NEED_RESCHED_LAZY which allows the >> scheduler to mark that a reschedule is needed, but is deferred until >> the task finishes executing in the kernel -- voluntary preemption >> as it were. >> >> The TIF_NEED_RESCHED flag is evaluated at all three of the preemption >> points. TIF_NEED_RESCHED_LAZY only needs to be evaluated at ret-to-user. >> >> ret-to-user ret-to-kernel preempt_count() >> none Y N N >> voluntary Y Y Y >> full Y Y Y > > Wait. The above is for when RESCHED_LAZY is to preempt, right? > > Then, shouldn't voluntary be: > > voluntary Y N N > > For LAZY, but > > voluntary Y Y Y > > For NEED_RESCHED (without lazy) Yes. You are, of course, right. I was talking about the TIF_NEED_RESCHED flags and in the middle switched to talking about how the voluntary model will get to what it wants. > That is, the only difference between voluntary and none (as you describe > above) is that when an RT task wakes up, on voluntary, it sets NEED_RESCHED, > but on none, it still sets NEED_RESCHED_LAZY? Yeah exactly. Just to restate without mucking it up: The TIF_NEED_RESCHED flag is evaluated at all three of the preemption points. TIF_NEED_RESCHED_LAZY only needs to be evaluated at ret-to-user. ret-to-user ret-to-kernel preempt_count() NEED_RESCHED_LAZY Y N N NEED_RESCHED Y Y Y Based on how various preemption models set the flag they would cause preemption at: ret-to-user ret-to-kernel preempt_count() none Y N N voluntary Y Y Y full Y Y Y >> The max-load numbers (not posted here) also behave similarly. > > It would be interesting to run any "latency sensitive" benchmarks. > > I wounder how cyclictest would work under each model with and without this > patch? Didn't post these numbers because I suspect that code isn't quite right, but voluntary preemption for instance does what it promises: # echo NO_FORCE_PREEMPT > sched/features # echo NO_PREEMPT_PRIORITY > sched/features # preempt=none # stress-ng --cyclic 1 --timeout 10 stress-ng: info: [1214172] setting to a 10 second run per stressor stress-ng: info: [1214172] dispatching hogs: 1 cyclic stress-ng: info: [1214174] cyclic: sched SCHED_DEADLINE: 100000 ns delay, 10000 samples stress-ng: info: [1214174] cyclic: mean: 9834.56 ns, mode: 3495 ns stress-ng: info: [1214174] cyclic: min: 2413 ns, max: 3145065 ns, std.dev. 77096.98 stress-ng: info: [1214174] cyclic: latency percentiles: stress-ng: info: [1214174] cyclic: 25.00%: 3366 ns stress-ng: info: [1214174] cyclic: 50.00%: 3505 ns stress-ng: info: [1214174] cyclic: 75.00%: 3776 ns stress-ng: info: [1214174] cyclic: 90.00%: 4316 ns stress-ng: info: [1214174] cyclic: 95.40%: 10989 ns stress-ng: info: [1214174] cyclic: 99.00%: 91181 ns stress-ng: info: [1214174] cyclic: 99.50%: 290477 ns stress-ng: info: [1214174] cyclic: 99.90%: 1360837 ns stress-ng: info: [1214174] cyclic: 99.99%: 3145065 ns stress-ng: info: [1214172] successful run completed in 10.00s # echo PREEMPT_PRIORITY > features # preempt=voluntary # stress-ng --cyclic 1 --timeout 10 stress-ng: info: [916483] setting to a 10 second run per stressor stress-ng: info: [916483] dispatching hogs: 1 cyclic stress-ng: info: [916484] cyclic: sched SCHED_DEADLINE: 100000 ns delay, 10000 samples stress-ng: info: [916484] cyclic: mean: 3682.77 ns, mode: 3185 ns stress-ng: info: [916484] cyclic: min: 2523 ns, max: 150082 ns, std.dev. 2198.07 stress-ng: info: [916484] cyclic: latency percentiles: stress-ng: info: [916484] cyclic: 25.00%: 3185 ns stress-ng: info: [916484] cyclic: 50.00%: 3306 ns stress-ng: info: [916484] cyclic: 75.00%: 3666 ns stress-ng: info: [916484] cyclic: 90.00%: 4778 ns stress-ng: info: [916484] cyclic: 95.40%: 5359 ns stress-ng: info: [916484] cyclic: 99.00%: 6141 ns stress-ng: info: [916484] cyclic: 99.50%: 7824 ns stress-ng: info: [916484] cyclic: 99.90%: 29825 ns stress-ng: info: [916484] cyclic: 99.99%: 150082 ns stress-ng: info: [916483] successful run completed in 10.01s This is with a background kernbench half-load. Let me see if I can dig out the numbers without this series. -- ankur