linux-kernel - Re: [PATCH 4/4 v0.4] sched/umcg: RFC: implement UMCG syscalls

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Message-ID: <3530714d-125b-e0f5-45b2-72695e2fc4ee@uwaterloo.ca>
Date:   Wed, 4 Aug 2021 18:04:57 -0400
From:   Thierry Delisle <tdelisle@...terloo.ca>
To:     <posk@...k.io>
CC:     <avagin@...gle.com>, <bsegall@...gle.com>, <jannh@...gle.com>,
        <linux-api@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <mingo@...hat.com>, <peterz@...radead.org>, <pjt@...gle.com>,
        <posk@...gle.com>, <tdelisle@...terloo.ca>, <tglx@...utronix.de>,
        Peter Buhr <pabuhr@...terloo.ca>
Subject: Re: [PATCH 4/4 v0.4] sched/umcg: RFC: implement UMCG syscalls

I have attached an atomic stack implementation I wrote. I believe it would
be applicable here. It is very similar except the kernel side no longer
needs a retry loop, the looping is moved to the user-space after the pop.
Using it instead of the code you have in enqueue_idle_worker means the
timeout is no longer needed.

 > - ``uint64_t idle_server_tid_ptr``: points to a pointer variable in the
 >   userspace that points to an idle server, i.e. a server in IDLE 
state waiting
 >   in sys_umcg_wait(); read-only; workers must have this field set; 
not used
 >   in servers.
 >
 >   When a worker's blocking operation in the kernel completes, the kernel
 >   changes the worker's state from ``BLOCKED`` to ``IDLE``, adds the 
worker
 >   to the list of idle workers, and checks whether
 >   ``*idle_server_tid_ptr`` is not zero. If not, the kernel tries to 
cmpxchg()
 >   it with zero; if cmpxchg() succeeds, the kernel will then wake the 
server.
 >   See `State transitions`_ below for more details.

In this case, I believe cmpxchg is not necessary and xchg suffices.


View attachment "atomic_stack.c" of type "text/x-csrc" (3151 bytes)