linux-kernel - Re: [PATCH 5/5 v0.6] sched/umcg: add Documentation/userspace-api/umcg.txt

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Date:   Wed, 22 Sep 2021 14:39:04 -0400
From:   Thierry Delisle <tdelisle@...terloo.ca>
To:     Peter Oskolkov <posk@...k.io>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        <linux-kernel@...r.kernel.org>, <linux-api@...r.kernel.org>
CC:     Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
        Peter Oskolkov <posk@...gle.com>,
        Andrei Vagin <avagin@...gle.com>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 5/5 v0.6] sched/umcg: add
 Documentation/userspace-api/umcg.txt

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