Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Fri, 4 Feb 2022 15:30:45 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Waiman Long <longman@redhat.com>,
        Sultan Alsawaf <sultan@kerneltoast.com>,
        Theodore Ts'o <tytso@mit.edu>,
        Andy Lutomirski <luto@kernel.org>,
        Jonathan =?utf-8?Q?Neusch=C3=A4fer?= <j.neuschaefer@gmx.net>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v2] random: remove batched entropy locking
Message-ID: <Yf04lcQIatnGspAb@linutronix.de>
References: <CAHmME9pe2BEJV4WiZNHmDmH_XK621Qqr1JCBdgTNZmr4JGBA4w@mail.gmail.com>
 <20220128223548.97807-1-Jason@zx2c4.com>
 <CAHmME9qtjZX2kVNSQqUsTrZv1cdR8y6n3yZS-RnpVCCzX9okcA@mail.gmail.com>
 <Yf0JlXf3ARsBpL9K@linutronix.de>
 <CAHmME9r0XxX3LqNLpVeqAjDQ_OVskPf15QOwxtZYy0tb_x_7HQ@mail.gmail.com>
 <Yf0xy4kZ2Mn65yp8@linutronix.de>
 <CAHmME9oOMhRVybTgHXT+oOXhMkdx7FVY7oSc-rHr=6AvZCVo=w@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <CAHmME9oOMhRVybTgHXT+oOXhMkdx7FVY7oSc-rHr=6AvZCVo=w@mail.gmail.com>
Precedence: bulk

On 2022-02-04 15:11:34 [+0100], Jason A. Donenfeld wrote:
> Hi Sebastian,
Hi Jason,

> On Fri, Feb 4, 2022 at 3:02 PM Sebastian Andrzej Siewior
> <bigeasy@linutronix.de> wrote:
> > The commit in tree you cited is b43db859a36cb553102c9c80431fc44618703bda.
> > It does not mention anything regarding faster nor the performance
> > improvement and conditions (hoth path, etc). It still has a stable tag.
> 
> It dropped the Cc: stable@. It still has the Fixes:. I can get rid of
> the Fixes: too. I'll improve that message a bunch for a potential v3.

Either you argue for bug fixing or performance improvement and I made it
clear that it is not bug fixing. That Fixes: tag is enough for Greg to
backport it.

> > > Maybe it'd be best to retain the spinlock_t, which will amount to
> > > disabling interrupts on !PREEMPT_RT, since it'll never be contended,
> > > but will turn into a mutex on PREEMPT_RT, where it'll do the right
> > > thing from an exclusivity perspective. Would this be reasonable?
> >
> > what does retain the spinlock_t mean since we already have a spinlock_t?
> 
> The idea would be to keep using spinlock_t like we do now -- no change
> there -- but move to using this atomic generation counter so that
> there's never any contention. Actually, though, I worry that that
> approach would throw out the gains we're getting by chucking the
> spinlock in the first place.

It is a per-CPU spinlock_t so there should be no contention if there is
no cross-CPU access. The overhead are two atomic operations.

> What if we keep a spinlock_t there on PREEMPT_RT but stick with
> disabling interrupts on !PREEMPT_RT? I wish there was a solution or an
> API that amounted to the same thing so there wouldn't need to be an
> #ifdef, but I don't know what that'd be.

If it is still to much try to look for locallock_t and
local_lock_irqsave(). This is kind of like your local_irq_save() but
you have lockdep annotations and PREEMPT_RT has a spinlock_t like
behaviour. It also documents in-code what the scope of your locking is.

> Jason

Sebastian