bpf/lpm_trie: Inline longest_prefix_match for fastpath

bpf/lpm_trie: Inline longest_prefix_match for fastpath

The BPF map type LPM (Longest Prefix Match) is used heavily
in production by multiple products that have BPF components.
Perf data shows trie_lookup_elem() and longest_prefix_match()
being part of kernels perf top.

For every level in the LPM tree trie_lookup_elem() calls out
to longest_prefix_match().  The compiler is free to inline this
call, but chooses not to inline, because other slowpath callers
(that can be invoked via syscall) exists like trie_update_elem(),
trie_delete_elem() or trie_get_next_key().

 bcc/tools/funccount -Ti 1 'trie_lookup_elem|longest_prefix_match.isra.0'
 FUNC                                    COUNT
 trie_lookup_elem                       664945
 longest_prefix_match.isra.0           8101507

Observation on a single random machine shows a factor 12 between
the two functions. Given an average of 12 levels in the trie being
searched.

This patch force inlining longest_prefix_match(), but only for
the lookup fastpath to balance object instruction size.

In production with AMD CPUs, measuring the function latency of
'trie_lookup_elem' (bcc/tools/funclatency) we are seeing an improvement
function latency reduction 7-8% with this patch applied (to production
kernels 6.6 and 6.1). Analyzing perf data, we can explain this rather
large improvement due to reducing the overhead for AMD side-channel
mitigation SRSO (Speculative Return Stack Overflow).

Fixes: fb3bd914b3ec ("x86/srso: Add a Speculative RAS Overflow mitigation")
Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/171076828575.2141737.18370644069389889027.stgit@firesoul