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Message-ID: <20240410230718.49778-1-kuniyu@amazon.com>
Date: Wed, 10 Apr 2024 16:07:18 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <willemdebruijn.kernel@...il.com>
CC: <davem@...emloft.net>, <krisman@...e.de>, <kuniyu@...zon.com>,
<lmb@...valent.com>, <martin.lau@...nel.org>, <netdev@...r.kernel.org>
Subject: Re: [PATCH v2] udp: Avoid call to compute_score on multiple sites
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Date: Wed, 10 Apr 2024 18:51:33 -0400
> 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;
TCP_ESTABLISHED never reaches the rescore jump as it's checked
before calling inet_lookup_reuseport() and inet_lookup_reuseport()
also does not select TCP_ESTABLISHED.
>
>
> >
> > > + 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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