linux-kernel - Re: [PATCH] rcu: use try_cmpxchg in check_cpu

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Message-ID: <CAEXW_YQ515_DOLVUm48GvDADuaY2mSrYTaKa7u6jYDNqBncJww@mail.gmail.com>
Date:   Tue, 28 Feb 2023 18:30:14 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Uros Bizjak <ubizjak@...il.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall

Hey Steve,

On Tue, Feb 28, 2023 at 4:41 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Tue, 28 Feb 2023 13:29:11 -0800
> "Paul E. McKenney" <paulmck@...nel.org> wrote:
>
> > All well and good, but the stall-warning code is nowhere near a fastpath.
> >
> > Is try_cmpxchg() considered more readable in this context?
>
>
> -           cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
> +           try_cmpxchg(&rcu_state.jiffies_stall, &js, jn)) {
>
> It's basically the same :-/
>
> But looking at this use case, I'd actually NAK it, as it is misleading.

I'm trying to parse this. You are saying it is misleading, because it
updates js when it doesn't need to?

> As try_cmpxchg() is used to get rid of the updating of the old value. As in
> the ring buffer code we had:
>
> void ring_buffer_record_off(struct trace_buffer *buffer)
> {
>         unsigned int rd;
>         unsigned int new_rd;
>
>         do {
>                 rd = atomic_read(&buffer->record_disabled);
>                 new_rd = rd | RB_BUFFER_OFF;
>         } while (!atomic_cmpxchg(&buffer->record_disabled, &rd, new_rd) != rd);

Hear you actually meant "rd" as the second parameter without the & ?

> }
>
> and the try_cmpxchg() converted it to:
>
> void ring_buffer_record_off(struct trace_buffer *buffer)
> {
>         unsigned int rd;
>         unsigned int new_rd;
>
>         rd = atomic_read(&buffer->record_disabled);
>         do {
>                 new_rd = rd | RB_BUFFER_OFF;
>         } while (!atomic_try_cmpxchg(&buffer->record_disabled, &rd, new_rd));
> }
>
> Which got rid of the need to constantly update the rd variable (cmpxchg
> will load rax with the value read, so it removes the need for an extra
> move).

So that's a good thing?

>
> But in your case, we don't need to update js, in which case the
> try_cmpxchg() does.

Right, it has lesser value here but I'm curious why you feel it also
doesn't belong in that ring buffer loop you shared (or did you mean,
it does belong there but not in other ftrace code modified by Uros?).

> The patch that Uros sent me for the ring buffer code also does some of
> that, which I feel is wrong.

I am guessing that is code *other than* the ring buffer loop above.

> So with that, I would nack the patch.

Dropping it for RCU is fine with me as well.

(And yes for the other thread, I had it backwards, try_cmpxchg is what
drops the need for cmp instruction -- sorry.)

thanks,

 -Joel