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 IkU6RYf7 NEoJRt1Ej7qkqs5T55eaF85SgjLllUQOGVA1MGh9X9phySFxejE3X2AvlOeKmNa68jhtEeUjBpm2m6cRRx/HSzItywZvN3MQfZuwbQrBURy4UFHdgRy9mxajrshIQU7GbSmm+14Kh8/Rup4U7/R0pkBDTQBR9KjGZVGvMdP0sJNwZSnZI0L5rTLbNkhU2FHY9vHpEJ4okDGUyWDZDfDP/eV8lJwXlWXnpSrM++ZAJcMwSE9VrnpcxymKOZ6OfaUbS/D3iudOGvU31KTuzgPrpTigw0As5Ae264TBTCr9Es9iU5DfnyJ9I9xVa/gv2Vc8xmZKKh6E6Sned1f9IyXhf067XNKdDIMpOalwssePJq8F/4opKF3xaC3DSQFpXTwEXsoh2BncY1cEZhCmhrVqjpn5oNrvWjazkeL39aavumhGplABUunbcniZcrzyBHpTfxOqiuqmkcD4ml+5A9slPuHXlgrbtKRxb61x/oXtY4++0kgI= 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: Thomas Gleixner writes: > So the decision matrix would be: > > Ret2user Ret2kernel PreemptCnt=0 > > NEED_RESCHED Y Y Y > LAZY_RESCHED Y N N > > That is completely independent of the preemption model and the > differentiation of the preemption models happens solely at the scheduler > level: This is relatively minor, but do we need two flags? Seems to me we can get to the same decision matrix by letting the scheduler fold into the preempt-count based on current preemption model. > PREEMPT_NONE sets only LAZY_RESCHED unless it needs to enforce the time > slice where it sets NEED_RESCHED. PREEMPT_NONE sets up TIF_NEED_RESCHED. For the time-slice expiry case, also fold into preempt-count. > PREEMPT_VOLUNTARY extends the NONE model so that the wakeup of RT class > tasks or sporadic event tasks sets NEED_RESCHED too. PREEMPT_NONE sets up TIF_NEED_RESCHED and also folds it for the RT/sporadic tasks. > PREEMPT_FULL always sets NEED_RESCHED like today. Always fold the TIF_NEED_RESCHED into the preempt-count. > We should be able merge the PREEMPT_NONE/VOLUNTARY behaviour so that we > only end up with two variants or even subsume PREEMPT_FULL into that > model because that's what is closer to the RT LAZY preempt behaviour, > which has two goals: > > 1) Make low latency guarantees for RT workloads > > 2) Preserve the throughput for non-RT workloads > > But in any case this decision happens solely in the core scheduler code > and nothing outside of it needs to be changed. > > So we not only get rid of the cond/might_resched() muck, we also get rid > of the static_call/static_key machinery which drives PREEMPT_DYNAMIC. > The only place which still needs that runtime tweaking is the scheduler > itself. True. The dynamic preemption could just become a scheduler tunable. > Though it just occured to me that there are dragons lurking: > > arch/alpha/Kconfig: select ARCH_NO_PREEMPT > arch/hexagon/Kconfig: select ARCH_NO_PREEMPT > arch/m68k/Kconfig: select ARCH_NO_PREEMPT if !COLDFIRE > arch/um/Kconfig: select ARCH_NO_PREEMPT > > So we have four architectures which refuse to enable preemption points, > i.e. the only model they allow is NONE and they rely on cond_resched() > for breaking large computations. > > But they support PREEMPT_COUNT, so we might get away with a reduced > preemption point coverage: > > Ret2user Ret2kernel PreemptCnt=0 > > NEED_RESCHED Y N Y > LAZY_RESCHED Y N N So from the discussion in the other thread, for the ARCH_NO_PREEMPT configs that don't support preemption, we probably need a fourth preemption model, say PREEMPT_UNSAFE. These could use only the Ret2user preemption points and just fallback to the !PREEMPT_COUNT primitives. Thanks -- ankur