On Tue, Oct 12, 2021 at 9:25 AM Thierry Delisle <tdelisle@xxxxxxxxxxxx> wrote: [...] > Just to be clear, sys_umcg_wait supports an operation that, when called > from > a worker, puts the worker to sleep without triggering block detection > or context-switching back to the server? Potentially, yes - when a worker wants to yield (e.g. as part of a custom UMCG-aware mutex/condvar code), and calls into the userspace scheduler, it may be faster to skip the server wakeup (e.g. reassign the server to another sleeping worker and wake this worker). This is not a supported operation right now, but I see how it could be used to optimize some things in the future. Do you have any concerns here? > > > > >> With that said, I'm a little confused by the usage of "yields" in that > >> example. I would expect workers yielding to behave like kernel threads > >> calling sched_yield(), i.e., context switch to the server but also be > >> immediately added to the idle_workers_ptr. > > > > I'm not a fan of arguing about how to name things. If the maintainers > > ask me to rename wait/wake to park/unpark, I'll do that. > > I understand the sentiment, and I'm perfectly happy with the use of > wait/wake. > I was exclusively referring to this one use of "yield" in the > documentation. I don't see a big difference here, sorry. We are mixing levels of abstraction here again, I think. For example, the higher level userspace scheduling code will have more nuanced treatment of IDLE workers; but down at the kernel they are all the same: IDLE worker is a worker that the userspace can "schedule" by marking it RUNNING, regardless of whether the worker is "parked", or "woke from a blocking op", or whatever other semantically different state the worker can be. For the kernel, they are all the same, idle, not runnable, waiting for the userspace to explicitly "schedule" them. Similarly, I don't see a need to semantically distinguish "yield" from "park" at the kernel level of things; this distinction seems to be a higher-level abstraction that can be properly expressed in the userspace, and does not need to be explicitly addressed in the kernel (to make the code faster and simpler, for example).
