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Message-ID: <661717f5c2839_2d123b294f@willemb.c.googlers.com.notmuch>
Date: Wed, 10 Apr 2024 18:51:33 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>,
krisman@...e.de
Cc: davem@...emloft.net,
lmb@...valent.com,
martin.lau@...nel.org,
netdev@...r.kernel.org,
willemdebruijn.kernel@...il.com,
Kuniyuki Iwashima <kuniyu@...zon.com>
Subject: Re: [PATCH v2] udp: Avoid call to compute_score on multiple sites
Kuniyuki Iwashima wrote:
> From: Gabriel Krisman Bertazi <krisman@...e.de>
> Date: Wed, 10 Apr 2024 17:50:47 -0400
> > We've observed a 7-12% performance regression in iperf3 UDP ipv4 and
> > ipv6 tests with multiple sockets on Zen3 cpus, which we traced back to
> > commit f0ea27e7bfe1 ("udp: re-score reuseport groups when connected
> > sockets are present"). The failing tests were those that would spawn
> > UDP sockets per-cpu on systems that have a high number of cpus.
> >
> > Unsurprisingly, it is not caused by the extra re-scoring of the reused
> > socket, but due to the compiler no longer inlining compute_score, once
> > it has the extra call site in udp4_lib_lookup2. This is augmented by
> > the "Safe RET" mitigation for SRSO, needed in our Zen3 cpus.
> >
> > We could just explicitly inline it, but compute_score() is quite a large
> > function, around 300b. Inlining in two sites would almost double
> > udp4_lib_lookup2, which is a silly thing to do just to workaround a
> > mitigation. Instead, this patch shuffles the code a bit to avoid the
> > multiple calls to compute_score. Since it is a static function used in
> > one spot, the compiler can safely fold it in, as it did before, without
> > increasing the text size.
> >
> > With this patch applied I ran my original iperf3 testcases. The failing
> > cases all looked like this (ipv4):
> > iperf3 -c 127.0.0.1 --udp -4 -f K -b $R -l 8920 -t 30 -i 5 -P 64 -O 2
> >
> > where $R is either 1G/10G/0 (max, unlimited). I ran 3 times each.
> > baseline is 6.9.0-rc1-g962490525cff, just a recent checkout of Linus
> > tree. harmean == harmonic mean; CV == coefficient of variation.
> >
> > ipv4:
> > 1G 10G MAX
> > HARMEAN (CV) HARMEAN (CV) HARMEAN (CV)
> > baseline 1730488.20(0.0050) 1639269.91(0.0795) 1436340.05(0.0954)
> > patched 1980936.14(0.0020) 1933614.06(0.0866) 1784184.51(0.0961)
> >
> > ipv6:
> > 1G 10G MAX
> > HARMEAN (CV) HARMEAN (CV) HARMEAN (CV)
> > baseline 1679016.07(0.0053) 1697504.56(0.0064) 1481432.74(0.0840)
> > patched 1924003.38(0.0153) 1852277.31(0.0457) 1690991.46(0.1848)
> >
> > This restores the performance we had before the change above with this
> > benchmark. We obviously don't expect any real impact when mitigations
> > are disabled, but just to be sure it also doesn't regresses:
> >
> > mitigations=off ipv4:
> > 1G 10G MAX
> > HARMEAN (CV) HARMEAN (CV) HARMEAN (CV)
> > baseline 3230279.97(0.0066) 3229320.91(0.0060) 2605693.19(0.0697)
> > patched 3242802.36(0.0073) 3239310.71(0.0035) 2502427.19(0.0882)
> >
> > Cc: Lorenz Bauer <lmb@...valent.com>
> > Fixes: f0ea27e7bfe1 ("udp: re-score reuseport groups when connected sockets are present")
> > Signed-off-by: Gabriel Krisman Bertazi <krisman@...e.de>
> >
> > ---
> > Changes since v1:
> > (me)
> > - recollected performance data after changes below only for the
> > mitigations enabled case.
> > (suggested by Willem de Bruijn)
> > - Drop __always_inline in compute_score
> > - Simplify logic by replacing third struct sock pointer with bool
> > - Fix typo in commit message
> > - Don't explicitly break out of loop after rescore
> > ---
> > net/ipv4/udp.c | 18 +++++++++++++-----
> > net/ipv6/udp.c | 17 +++++++++++++----
> > 2 files changed, 26 insertions(+), 9 deletions(-)
> >
> > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > index 661d0e0d273f..a13ef8e06093 100644
> > --- a/net/ipv4/udp.c
> > +++ b/net/ipv4/udp.c
> > @@ -427,12 +427,15 @@ static struct sock *udp4_lib_lookup2(struct net *net,
> > {
> > struct sock *sk, *result;
> > int score, badness;
> > + bool rescore = false;
>
> nit: Keep reverse xmax tree order.
> https://docs.kernel.org/process/maintainer-netdev.html#local-variable-ordering-reverse-xmas-tree-rcs
>
> >
> > result = NULL;
> > badness = 0;
> > udp_portaddr_for_each_entry_rcu(sk, &hslot2->head) {
> > - score = compute_score(sk, net, saddr, sport,
> > - daddr, hnum, dif, sdif);
> > +rescore:
> > + score = compute_score((rescore ? result : sk), net, saddr,
>
> I guess () is not needed around rescore ?
>
> Both same for IPv6.
>
> Otherwise, looks good to me.
>
> Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>
Can we avoid using the same name for the label and boolean?
And since if looping result will have state TCP_ESTABLISHED, can it
just be
sk = result;
goto rescore;
>
> > + sport, daddr, hnum, dif, sdif);
> > + rescore = false;
> > if (score > badness) {
> > badness = score;
> >
> > @@ -456,9 +459,14 @@ static struct sock *udp4_lib_lookup2(struct net *net,
> > if (IS_ERR(result))
> > continue;
> >
> > - badness = compute_score(result, net, saddr, sport,
> > - daddr, hnum, dif, sdif);
> > -
> > + /* compute_score is too long of a function to be
> > + * inlined, and calling it again here yields
> > + * measureable overhead for some
> > + * workloads. Work around it by jumping
> > + * backwards to rescore 'result'.
> > + */
> > + rescore = true;
> > + goto rescore;
> > }
> > }
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