On 15/03/2024 16.03, Daniel Borkmann wrote:
On 3/12/24 4:17 PM, Jesper Dangaard Brouer wrote:
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.
You mention these are heavy hitters in prod ...
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 metal 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.
$ bloat-o-meter kernel/bpf/lpm_trie.o.orig-noinline
kernel/bpf/lpm_trie.o
add/remove: 1/1 grow/shrink: 1/0 up/down: 335/-4 (331)
Function old new delta
trie_lookup_elem 179 510 +331
__BTF_ID__struct__lpm_trie__706741 - 4 +4
__BTF_ID__struct__lpm_trie__706733 4 - -4
Total: Before=3056, After=3387, chg +10.83%
... and here you quote bloat-o-meter instead. But do you also see an
observable perf gain in prod after this change? (No objection from my
side but might be good to mention here.. given if not then why do the
change?)
I'm still waiting for more production servers to reboot into patched
kernels. I do have some "low-level" numbers from previous generation
AMD servers, running kernel 6.1, which should be less affected by the
SRSO (than our 6.6 kernels). Waiting for newer generation to get kernel
updates, and especially 6.6 will be interesting.
From production measurements the latency overhead of trie_lookup_elem:
- avg 1220 nsecs for patched kernel
- avg 1329 nsecs for non patched kernel
- around 8% improvement or 109 nanosec
- given approx 12 calls "saved" this is 9 ns per function call
- for reference on Intel I measured func call to cost 1.3ns
- this extra overhead is caused by __x86_return_thunk().
I also see slight improvement in the graphs, but given how much
production varies I don't want to draw conclusions yet.
Details: Due to AMD mitigation for SRSO (Speculative Return Stack
Overflow)
these function calls have additional overhead. On newer kernels this
shows
up under srso_safe_ret() + srso_return_thunk(), and on older kernels
(6.1)
under __x86_return_thunk(). Thus, for production workloads the biggest
gain
comes from avoiding this mitigation overhead.
Signed-off-by: Jesper Dangaard Brouer <hawk@xxxxxxxxxx>
