Back in 2024 I reported a 7-12% regression on an iperf3 UDP loopback
thoughput test that we traced to the extra overhead of calling
compute_score on two places, introduced by commit f0ea27e7bf ("udp:
re-score reuseport groups when connected sockets are present"). At the
time, I pointed out the overhead was caused by the multiple calls,
associated with cpu-specific mitigations, and merged commit
50aee97d15 ("udp: Avoid call to compute_score on multiple sites") to
jump back explicitly, to force the rescore call in a single place.
Recently though, we got another regression report against a newer distro
version, which a team colleague traced back to the same root-cause.
Turns out that once we updated to gcc-13, the compiler got smart enough
to unroll the loop, undoing my previous mitigation. Let's bite the
bullet and __always_inline compute_score on both ipv4 and ipv6 to
prevent gcc from de-optimizing it again in the future. These functions
are only called in two places each, udpX_lib_lookup1 and
udpX_lib_lookup2, so the extra size shouldn't be a problem and it is hot
enough to be very visible in profilings. In fact, with gcc13, forcing
the inline will prevent gcc from unrolling the fix from commit
50aee97d15, so we don't end up increasing udpX_lib_lookup2 at all.
I haven't recollected the results myself, as I don't have access to the
machine at the moment. But the same colleague reported 4.67%
inprovement with this patch in the loopback benchmark, solving the
regression report within noise margins.
Eric Dumazet reported no size change to vmlinux when built with clang.
I report the same also with gcc-13:
scripts/bloat-o-meter vmlinux vmlinux-inline
add/remove: 0/2 grow/shrink: 4/0 up/down: 616/-416 (200)
Function old new delta
udp6_lib_lookup2 762 949 +187
__udp6_lib_lookup 810 975 +165
udp4_lib_lookup2 757 906 +149
__udp4_lib_lookup 871 986 +115
__pfx_compute_score 32 - -32
compute_score 384 - -384
Total: Before=35011784, After=35011984, chg +0.00%
Fixes: 50aee97d15 ("udp: Avoid call to compute_score on multiple sites")
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
Link: https://patch.msgid.link/20260410155936.654915-1-krisman@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
b80a95ccf1