On Wed, 17 Jul 2024 10:36:47 -0400 Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > The problem with that is the only use case for such a feature is for > vCPUS. There's no use case for a single thread to up and down its > priority. I work a lot in RT applications (well, not as much anymore, > but my career was heavy into it). And I can't see any use case where a > single thread would bounce its priority around. In fact, if I did see > that, I would complain that it was a poorly designed system. > > Now for a guest kernel, that's very different. It has to handle things > like priority inheritance and such, where bouncing a threads (or its > own vCPU thread) priority most definitely makes sense. > > So you are requesting that we add a bad user space interface to allow > lazy priority management from a thread so that we can use it in the > proper use case of a vCPU? Now I stated the above thinking you wanted to add a generic interface for all user space. But perhaps there is a way to get this to be done by the scheduler itself. But its use case is still only for VMs. We could possibly add a new sched class that has a dynamic priority. That is, it can switch between other sched classes. A vCPU thread could be assigned to this class from inside the kernel (via a virtio device) where this is not exposed to user space at all. Then the virtio device would control the mapping of a page between the vCPU thread and the host kernel. When this task gets scheduled, it can call into the code that handles the dynamic priority. This will require buy-in from the scheduler folks. This could also handle the case of a vCPU being woken up by an interrupt, as the hooks could be there on the wakeup side as well. Thoughts? -- Steve
