netdev - Re: [PATCH net-next] once: add DO_ONCE

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Date:   Sun, 2 Oct 2022 00:50:38 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Willy Tarreau <w@....eu>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Subject: Re: [PATCH net-next] once: add DO_ONCE_SLOW() for sleepable contexts

On Sat, Oct 01, 2022 at 11:15:29PM +0200, Willy Tarreau wrote:
> Hi Eric,
> 
> On Sat, Oct 01, 2022 at 01:51:02PM -0700, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> > 
> > Christophe Leroy reported a ~80ms latency spike
> > happening at first TCP connect() time.
> 
> Seeing Christophe's message also made me wonder if we didn't break
> something back then :-/
> 
> > This is because __inet_hash_connect() uses get_random_once()
> > to populate a perturbation table which became quite big
> > after commit 4c2c8f03a5ab ("tcp: increase source port perturb table to 2^16")
> > 
> > get_random_once() uses DO_ONCE(), which block hard irqs for the duration
> > of the operation.
> > 
> > This patch adds DO_ONCE_SLOW() which uses a mutex instead of a spinlock
> > for operations where we prefer to stay in process context.
> 
> That's a nice improvement I think. I was wondering if, for this special
> case, we *really* need an exclusive DO_ONCE(). I mean, we're getting
> random bytes, we really do not care if two CPUs change them in parallel
> provided that none uses them before the table is entirely filled. Thus
> that could probably end up as something like:
> 
>     if (!atomic_read(&done)) {
>         get_random_bytes(array);
>         atomic_set(&done, 1);
>     }

If you don't care about the tables being consistent between CPUs, then
yea, sure, that seems like a reasonable approach, and I like not
polluting once.{c,h} with some _SLOW() special cases. If you don't want
the atomic read in there you could also do the same pattern with a
static branch, like what DO_ONCE() does:

   if (static_branch_unlikely(&need_bytes)) {
      get_random_bytes(array);
      static_branch_disable(&need_bytes);
   }

Anyway, same thing as your suggestion more or less.

Jason