On Mon, 10 Feb 2025 09:07:27 -0500 Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > On Thu, Feb 6, 2025 at 8:30 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > > > On Wed, 5 Feb 2025 22:07:12 -0500 > > Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > > > > > > > RT tasks don't have a time slice. They are affected by events. An external > > > > interrupt coming in, or a timer going off that states something is > > > > happening. Perhaps we could use this for SCHED_RR or maybe even > > > > SCHED_DEADLINE, as those do have time slices. > > > > > > > > But if it does get used, it should only be used when the task being > > > > scheduled is the same SCHED_RR priority, or if SCHED_DEADLINE will not fail > > > > its guarantees. > > > > > > > > > > Right, it would apply still to RR/DL though... > > > > But it would have to guarantee that the RR it is delaying is of the same > > priority, and that delaying the DL is not going to cause something to miss > > its deadline. > > See Peter comment: "Then pick another number; RT too has a max > scheduling latency number (on some random hardware). If you stay below > that, all is fine.". See mine and Sebastion's reply. Just adding another interrupt turn around, and you already lost. > > You see, LAZY was *created* for this purpose. Of letting the scheduler know > > that the running task is in a critical section and the timer tick should > > not preempt a SCHED_OTHER task. > > I just wanted to extend this to SCHED_OTHER in user space too. > > Currently it does not "let anyone know" it is running in a critical > section though. Various paths (update_curr(), wake up) just do a > 'lazy' resched until the timer tick has elapsed, or the task returns > to usermode/idle at which point schedule() is called. And it does this > only for FAIR tasks. That can well happen even if the currently > running task is not in a critical section in the kernel at all. Sure, > it may benefit critical sections in the upstream kernel but where is > that explicit? I still feel we should not overload this in-kernel > mechanism for userspace locking and complicate things. That's nice that you don't feel it. But I do. ;-) > > > > Yes, I have worked on RT projects before -- you would know better > > > than anyone. :-D. But admittedly, I haven't got to work much with > > > PREEMPT_RT systems. > > > > Just using RT policy to improve performance is not an RT project. I'm > > talking about projects that if you miss a deadline things crash. Where the > > project works very hard to make sure everything works as intended. > > No no no, I have done way more than applying just the RT policy. So > that means you do not know me that well;-).. I have worked on audio > driver latency, low latency audio, latency issues in vmalloc code, > preempt tracers, irq tracepoints , wake up latency tracers and various > scheduler overhead debug — many of those issues dealt with sub > millisecond latency.. I also dealt with cpu idle issues in the > hardware causing real time latency problems (see my past talks if > interested). I was partly a hardware engineer when I started my > career and have built circuits. I have Electronics and Computer > engineering degrees. None of that sounds to me like an RT project. I'm talking about robotics, industrial machines, autonomous driving, power plants, etc. Where everything is measured in WCET. -- Steve
