linux-kernel - Re: [PATCH v2] random: remove batched entropy locking

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Date:   Fri, 4 Feb 2022 15:30:45 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Theodore Ts'o <tytso@....edu>,
        Andy Lutomirski <luto@...nel.org>,
        Jonathan Neuschäfer <j.neuschaefer@....net>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] random: remove batched entropy locking

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@...utronix.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