Re: [PATCH bpf-next] bpf/lpm_trie: inline longest_prefix_match for fastpath

[Jesper Dangaard Brouer <hawk@kernel.org>]

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@kernel.org>