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Message-ID: <20240411000807.55368-1-kuniyu@amazon.com>
Date: Wed, 10 Apr 2024 17:08:07 -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 19:18:14 -0400
> Kuniyuki Iwashima wrote:
> > 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
Sorry I forgot about BPF. I think BPF can select established one,
so this part is not true.
> > before calling inet_lookup_reuseport() and inet_lookup_reuseport()
> > also does not select TCP_ESTABLISHED.
>
> I was thinking of the second part, inet_lookup_reuseport returning
> a connection from the group. I suppose this is assured not to be
> the case due to
>
> /* Falleback to scoring if grnult;p has connections */
> if (!reuseport_has_conns(sk))
> return result;
>
>
> Is that what you mean?
reuseport_has_conns() is for reuseport group mixed with TCP_CLOSE and
TCP_ESTABLISHED, and here sk is usually (I mean without BPF) TCP_CLOSE
so that we don't decide too early and can check TCP_ESTABLISHED socket
placed later in the same hash chain.
Also, reuseport_select_sock_by_hash() returns NULL If the reuseport group
has only TCP_ESTABLISHED sockets when selecting, and then we continue with
result = sk;
>
> There are a lot of hidden assumptions then to make sure this
> control flow is correct.
>
> Should we instead just have
>
> badness = score;
>
> + if (rescore)
> + continue;
The 2nd compute_score() is added recently for a situation where
inet_lookup_reuseport() returns sk with higher score to avoid
calling inet_lookup_reuseport() again for the result.
f0ea27e7bfe1 ("udp: re-score reuseport groups when connected sockets are present")
>
> Also, can the rescore = false in the datapath be avoided. The purpose
> is to only jump once. It would be good if it is obvious that a
> repeated (or infinite) loop is not possible, regardless of what
> the callees return.
>
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