linux-kernel - Re: [PATCH RFC v2 rcu 1/8] srcu: Convert ->srcu_lock_count and ->srcu_unlock

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Message-ID: <87wn9k7g4o.fsf@jogness.linutronix.de>
Date:   Fri, 30 Sep 2022 22:43:43 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH RFC v2 rcu 1/8] srcu: Convert ->srcu_lock_count and
 ->srcu_unlock_count to atomic

On 2022-09-30, "Paul E. McKenney" <paulmck@...nel.org> wrote:
>> > -	this_cpu_inc(ssp->sda->srcu_lock_count[idx]);
>> > +	this_cpu_inc(ssp->sda->srcu_lock_count[idx].counter);
>> 
>> Is there any particular reason that you are directly modifying
>> @counter instead of raw_cpu_ptr()+atomic_long_inc() that do you in
>> __srcu_read_lock_nmisafe() of patch 2?
>
> Performance.  From what I can see, this_cpu_inc() is way faster than
> atomic_long_inc() on x86 and s390.  Maybe also on loongarch.  No idea
> on arm64.

Yeah, that's what I figured. I just wanted to make sure.

FWIW, the rest of the series looks pretty straight forward to me.

John