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

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Date:   Wed, 4 Aug 2021 16:30:05 -0700
From:   Peter Oskolkov <posk@...gle.com>
To:     Thierry Delisle <tdelisle@...terloo.ca>
Cc:     posk@...k.io, 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, tglx@...utronix.de,
        Peter Buhr <pabuhr@...terloo.ca>
Subject: Re: [PATCH 4/4 v0.4] sched/umcg: RFC: implement UMCG syscalls

On Wed, Aug 4, 2021 at 3:05 PM Thierry Delisle <tdelisle@...terloo.ca> wrote:
>
> 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.

Thanks, Thierry! Your implementation seems reasonable - I'll test it
and will use it in the future versions of the patchset if everything
is OK.

>
>  > - ``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.

Right! I'll need to roll another atomic_xchg helper for this and the
atomic stack above...