Messages in this thread | | | Subject | Re: [PATCH 5/5 v0.6] sched/umcg: add Documentation/userspace-api/umcg.txt | From | Thierry Delisle <> | Date | Wed, 22 Sep 2021 14:39:04 -0400 |
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On 2021-09-17 2:03 p.m., Peter Oskolkov wrote: > [...] > +SYS_UMCG_WAIT() > + > +int sys_umcg_wait(uint32_t flags, uint64_t abs_timeout) operates on > +registered UMCG servers and workers: struct umcg_task *self provided to > +sys_umcg_ctl() when registering the current task is consulted in addition > +to flags and abs_timeout parameters. > + > +The function can be used to perform one of the three operations: > + > +* wait: if self->next_tid is zero, sys_umcg_wait() puts the current > + server or worker to sleep;
I believe this description is misleading but I might be wrong. From the example * worker to server context switch (worker "yields"): S:IDLE+W:RUNNING => +S:RUNNING+W:IDLE
It seems to me that when a worker goes from running to idle, it should *not* set the next_tid to 0, it should preserve the next_tid as-is, which is expected to point to its current server. This is consistent with my understanding of the umcg_wait implementation. This operation is effectively a direct context-switch to the server.
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.
From my understanding of the umcg_wait call, "worker to server context switch" does not have analogous behaviour to sched_yield. Am I correct? If so, I suggest using "park" instead of "yield" in the description of that example. I believe the naming of wait/wake as park/unpark is consistent with Java[1] and Rust[2], but I don't know if that naming is used in contexts closer to the linux kernel.
[1] https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/LockSupport.html [2] https://doc.rust-lang.org/std/thread/fn.park.html
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