On Tue, Nov 21, 2023 at 10:00:59AM -0500, Steven Rostedt wrote: > On Mon, 20 Nov 2023 21:04:28 -0800 > "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote: > > > How about like this, where "Y" means allowed and "N" means not allowed: > > > > Non-Preemptible RCU Preemptible RCU > > > > NONE: Y Y > > > > VOLUNTARY: Y Y > > > > PREEMPT: N Y > > > > PREEMPT_RT: N Y > > > > > > We need preemptible RCU for NONE and VOLUNTARY, as you say, > > to allow CONFIG_PREEMPT_DYNAMIC to continue to work. (OK, OK, > > CONFIG_PREEMPT_DYNAMIC is no longer, but appears to be unconditional.) > > But again, I don't see why anyone would want (much less need) > > non-preemptible RCU in the PREEMPT and PREEMPT_RT cases. And if it is > > neither wanted nor needed, there is no point in enabling it, much less > > testing it. > > > > Or am I missing a use case in there somewhere? > > As Ankur replied, this is just an RFC, not the main goal. I'm talking about > the end product which will get rid of the PREEMPT_NONE, PREEMPT_VOLUNTARY > and PREEMPT conifgs, and there will *only* be the PREEMPT_DYNAMIC and > PREEMPT_RT. > > And yes, this is going to be a slow and long processes, to find and fix all > regressions. I too am concerned about the latency that this may add. I'm > thinking we could have NEED_RESCHED_LAZY preempt when there is no mutex or > other semi critical section held (like migrate_disable()). Indeed. For one thing, you have a lot of work to do to demonstrate that this would actually be a good thing. For example, what is so horribly bad about selecting minimal preemption (NONE and/or VOLUNTARY) at build time??? > Right now, the use of cond_resched() is basically a whack-a-mole game where > we need to whack all the mole loops with the cond_resched() hammer. As > Thomas said, this is backwards. It makes more sense to just not preempt in > areas that can cause pain (like holding a mutex or in an RCU critical > section), but still have the general kernel be fully preemptable. Which is quite true, but that whack-a-mole game can be ended without getting rid of build-time selection of the preemption model. Also, that whack-a-mole game can be ended without eliminating all calls to cond_resched(). Additionally, if the end goal is to be fully preemptible as in eventually eliminating lazy preemption, you have a lot more convincing to do. For but one example, given the high cost of the additional context switches that will visit on a number of performance-sensitive workloads. So what exactly are you guys trying to accomplish here? ;-) Thanx, Paul
