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The way I > understand is that you make a change in 3/6 and then all other patches > in this series are required to deal with this. > > For bisect reasons it would make sense to have 3/6 last in the series > and to the "fixes" first before the code is enabled. I mean if you apply > 3/6 first then you get build failures without 1/6. But with 3/6 before > 5/6 you should get runtime errors, right? That's a good point. Will reorder. >> The main problem addressed in the RCU related patches is that before >> PREEMPT_LAZY, PREEMPTION=y implied PREEMPT_RCU=y. With PREEMPT_LAZY, >> that's no longer true. > > No, you want to make PREEMPTION=y + PREEMPT_RCU=n + PREEMPT_LAZY=y > possible. This is different. Your wording makes it sound like there _is_ > an actual problem. That's too literal a reading. It's just the problem ("matter or situation that is unwelcome" to quote from a dictionary) addressed in the patches. >> That's because PREEMPT_RCU makes some trade-offs to optimize for >> latency as opposed to throughput, and configurations with limited >> preemption might prefer the stronger forward-progress guarantees of >> PREEMPT_RCU=n. >> >> Accordingly, with standalone PREEMPT_LAZY (much like PREEMPT_NONE, >> PREEMPT_VOLUNTARY) we want to use PREEMPT_RCU=n. And, when used in >> conjunction with PREEMPT_DYNAMIC, we continue to use PREEMPT_RCU=y. >> >> Patches 1 and 2 are cleanup patches: >> "rcu: fix header guard for rcu_all_qs()" >> "rcu: rename PREEMPT_AUTO to PREEMPT_LAZY" >> >> Patch 3, "rcu: limit PREEMPT_RCU configurations", explicitly limits >> PREEMPT_RCU=y to the PREEMPT_DYNAMIC or the latency oriented models. >> >> Patches 4 and 5, >> "rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y" >> "osnoise: handle quiescent states for PREEMPT_RCU=n, PREEMPTION=y" >> >> handle quiescent states for the (PREEMPT_LAZY=y, PREEMPT_RCU=n) >> configuration. > > I was briefly thinking about > > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -5646,8 +5646,11 @@ void sched_tick(void) > hw_pressure = arch_scale_hw_pressure(cpu_of(rq)); > update_hw_load_avg(rq_clock_task(rq), rq, hw_pressure); > > - if (dynamic_preempt_lazy() && tif_test_bit(TIF_NEED_RESCHED_LAZY)) > + if (dynamic_preempt_lazy() && tif_test_bit(TIF_NEED_RESCHED_LAZY)) { > resched_curr(rq); > + if (!IS_ENABLED(CONFIG_PREEMPT_RCU)) > + rcu_all_qs(); > + } > > donor->sched_class->task_tick(rq, donor, 0); > if (sched_feat(LATENCY_WARN)) > > which should make #4+ #5 obsolete. But I think it is nicer to have the > change in #4 since it extends the check to cover all cases. And then > we would do it twice just for osnoise. Yeah, exactly. The check here only deals with this specific case while the one in rcu_flavor_sched_clock_irq() can handle that more generally. Thanks. -- ankur