netdev - Re: [PATCH net] net: avoid indirect memory pressure calls

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Message-ID: <CANn89i+jO1Zbd5w_sCO3muvo=jByWOaXxd9EUjHZiYJSYAATBw@mail.gmail.com>
Date:   Tue, 28 Feb 2023 17:35:49 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Alexander Lobakin <aleksander.lobakin@...el.com>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        davem@...emloft.net, pabeni@...hat.com, shakeelb@...gle.com,
        soheil@...gle.com
Subject: Re: [PATCH net] net: avoid indirect memory pressure calls

On Tue, Feb 28, 2023 at 5:35 PM Florian Westphal <fw@...len.de> wrote:
>
> Alexander Lobakin <aleksander.lobakin@...el.com> wrote:
> > From: Jakub Kicinski <kuba@...nel.org>
> > Date: Mon, 27 Feb 2023 15:27:41 -0800
> >
> > > On Fri, 24 Feb 2023 19:46:06 +0100 Florian Westphal wrote:
> > >> There is a noticeable tcp performance regression (loopback or cross-netns),
> > >> seen with iperf3 -Z (sendfile mode) when generic retpolines are needed.
> > >>
> > >> With SK_RECLAIM_THRESHOLD checks gone number of calls to enter/leave
> > >> memory pressure happen much more often. For TCP indirect calls are
> > >> used.
> > >>
> > >> We can't remove the if-set-return short-circuit check in
> > >> tcp_enter_memory_pressure because there are callers other than
> > >> sk_enter_memory_pressure.  Doing a check in the sk wrapper too
> > >> reduces the indirect calls enough to recover some performance.
> > >>
> > >> Before,
> > >> 0.00-60.00  sec   322 GBytes  46.1 Gbits/sec                  receiver
> > >>
> > >> After:
> > >> 0.00-60.04  sec   359 GBytes  51.4 Gbits/sec                  receiver
> > >>
> > >> "iperf3 -c $peer -t 60 -Z -f g", connected via veth in another netns.
> > >>
> > >> Fixes: 4890b686f408 ("net: keep sk->sk_forward_alloc as small as possible")
> > >> Signed-off-by: Florian Westphal <fw@...len.de>
> > >
> > > Looks acceptable, Eric?
> > >
> > I'm no Eric, but I'd only change this:
> >
> > +     if (!memory_pressure || READ_ONCE(*memory_pressure) == 0)
> >
> > to
> >
> > +     if (!memory_pressure || !READ_ONCE(*memory_pressure))
>
> I intentioanlly used '== 0', i found it too easy to miss the '!' before
> 'R'.  But maybe I just need better glasses.

Sorry for the delay, I will take a look.