Date:   Wed, 1 Mar 2023 13:18:31 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Uros Bizjak <ubizjak@gmail.com>
Cc:     linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Masami Hiramatsu <mhiramat@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [PATCH 3/3] ring_buffer: Use try_cmpxchg instead of cmpxchg
Message-ID: <20230301131831.6c8d4ff5@gandalf.local.home>
In-Reply-To: <CAFULd4YePGAqxpq0kJMnNHkqqkL9HEnEz1ud9kNrae5wqpRVbA@mail.gmail.com>
References: <20230228175929.7534-1-ubizjak@gmail.com>
        <20230228175929.7534-4-ubizjak@gmail.com>
        <20230228164346.0691bb11@gandalf.local.home>
        <CAFULd4aGx=kGYYUz0BkFJz3abz97WOwDEgFHAOocT8SWT2oX-Q@mail.gmail.com>
        <20230301111850.768f8526@gandalf.local.home>
        <CAFULd4YePGAqxpq0kJMnNHkqqkL9HEnEz1ud9kNrae5wqpRVbA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Wed, 1 Mar 2023 18:16:04 +0100
Uros Bizjak <ubizjak@gmail.com> wrote:

> > It's different, but I'm not so sure it's beneficial.  
> 
> This is the place we are looking for. Please see that a move at $1464
> and a compare at $1473 is missing in the assembly from the patched
> code. If it is beneficial ... well, we achieved the same result with
> two instructions less in a loopy code.

Note, none of these locations are in fast paths. (now if we had a
local_try_cmpxchg() then we could improve those locations!).

Thus my main concern here is for correctness. Unfortunately, to add a
cmpxchg_success() would require a lot of work, as all the cmpxchg()
functions are really macros that do a lot of magic (the
include/linux/atomic/atomic-instrumented.h says its even generated!). So
that will likely not happen.

I have mixed feelings for this patch. I like the fact that you are looking
in optimizing the code, but I'm also concerned that it could cause issues
later down the road.

I am leaning to just taking this, and hope that it doesn't cause problems.
And I would really love to change all the local_cmpxchg() to
local_try_cmpxchg(). Hmm, I wonder how much of an effort that would be to
implement those?

-- Steve