Re: [PATCH v2] udp: Avoid call to compute_score on multiple sites

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Wed, 10 Apr 2024 19:18:14 -0400
> Kuniyuki Iwashima wrote:
> > From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> > Date: Wed, 10 Apr 2024 18:51:33 -0400
> > > Kuniyuki Iwashima wrote:
> > > > From: Gabriel Krisman Bertazi <krisman@suse.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@isovalent.com>
> > > > > Fixes: f0ea27e7bfe1 ("udp: re-score reuseport groups when connected sockets are present")
> > > > > Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.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@amazon.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.
>