bcm53xx: use -falign-functions=32 for kernel compilation
authorRafał Miłecki <rafal@milecki.pl>
Sun, 3 Jul 2022 11:22:00 +0000 (13:22 +0200)
committerRafał Miłecki <rafal@milecki.pl>
Fri, 8 Jul 2022 14:18:25 +0000 (16:18 +0200)
commitb54ef39e0b910a4b8eaca0497fe9b63e8392262a
treeb9ed5ba53325ebdfc918af0f5b3d62d6b94f6d0e
parenta50f5b3fb2024fe00f726dff2093a83410bd3c8d
bcm53xx: use -falign-functions=32 for kernel compilation

Northstar SoCs have pretty small CPU caches and their performance is
heavily affected by cache hits & misses. It means that all kind of
random code changes can affect performance as they often reorganize
(change alignment & possibly reorder) kernel symbols.

It was discussed in ARM / net mailinglists:
1. ARM router NAT performance affected by random/unrelated commits [1] [2]
2. Optimizing kernel compilation / alignments for network performance [3] [4]

It seems that -falign-functions can be used as a partial workaround. It
doesn't solve all cases (e.g. documented watchdog one [5]) but it surely
helps with many of them.

A complete long term solution may be PGO (profile-guided optimization)
but it isn't available at this point.

[1] https://lkml.org/lkml/2019/5/21/349
[2] https://www.spinics.net/lists/linux-block/msg40624.html
[3] https://lore.kernel.org/linux-arm-kernel/066fc320-dc04-11a4-476e-b0d11f3b17e6@gmail.com/T/
[4] https://www.spinics.net/lists/netdev/msg816103.html
[5] http://lists.openwrt.org/pipermail/openwrt-devel/2022-July/038989.html

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit abc5b28db164dc2d807750cb2baae91e288c84a9)
config/Config-devel.in