On Sat, Oct 10, 2020 at 4:40 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> Recent work in f4d05259213f ("bpf: Add map_meta_equal map ops") and 134fede4eecf
> ("bpf: Relax max_entries check for most of the inner map types") added support
> for dynamic inner max elements for most map-in-map types. Exceptions were maps
> like array or prog array where the map_gen_lookup() callback uses the maps'
> max_entries field as a constant when emitting instructions.
>
> We recently implemented Maglev consistent hashing into Cilium's load balancer
> which uses map-in-map with an outer map being hash and inner being array holding
> the Maglev backend table for each service. This has been designed this way in
> order to reduce overall memory consumption given the outer hash map allows to
> avoid preallocating a large, flat memory area for all services. Also, the
> number of service mappings is not always known a-priori.
>
> The use case for dynamic inner array map entries is to further reduce memory
> overhead, for example, some services might just have a small number of back
> ends while others could have a large number. Right now the Maglev backend table
> for small and large number of backends would need to have the same inner array
> map entries which adds a lot of unneeded overhead.
>
> Dynamic inner array map entries can be realized by avoiding the inlined code
> generation for their lookup. The lookup will still be efficient since it will
> be calling into array_map_lookup_elem() directly and thus avoiding retpoline.
> The patch adds a BPF_F_INNER_MAP flag to map creation which therefore skips
> inline code generation and relaxes array_map_meta_equal() check to ignore both
> maps' max_entries. This also still allows to have faster lookups for map-in-map
> when BPF_F_INNER_MAP is not specified and hence dynamic max_entries not needed.
>
> Example code generation where inner map is dynamic sized array:
>
> # bpftool p d x i 125
> int handle__sys_enter(void * ctx):
> ; int handle__sys_enter(void *ctx)
> 0: (b4) w1 = 0
> ; int key = 0;
> 1: (63) *(u32 *)(r10 -4) = r1
> 2: (bf) r2 = r10
> ;
> 3: (07) r2 += -4
> ; inner_map = bpf_map_lookup_elem(&outer_arr_dyn, &key);
> 4: (18) r1 = map[id:468]
> 6: (07) r1 += 272
> 7: (61) r0 = *(u32 *)(r2 +0)
> 8: (35) if r0 >= 0x3 goto pc+5
> 9: (67) r0 <<= 3
> 10: (0f) r0 += r1
> 11: (79) r0 = *(u64 *)(r0 +0)
> 12: (15) if r0 == 0x0 goto pc+1
> 13: (05) goto pc+1
> 14: (b7) r0 = 0
> 15: (b4) w6 = -1
> ; if (!inner_map)
> 16: (15) if r0 == 0x0 goto pc+6
> 17: (bf) r2 = r10
> ;
> 18: (07) r2 += -4
> ; val = bpf_map_lookup_elem(inner_map, &key);
> 19: (bf) r1 = r0 | No inlining but instead
> 20: (85) call array_map_lookup_elem#149280 | call to array_map_lookup_elem()
> ; return val ? *val : -1; | for inner array lookup.
> 21: (15) if r0 == 0x0 goto pc+1
> ; return val ? *val : -1;
> 22: (61) r6 = *(u32 *)(r0 +0)
> ; }
> 23: (bc) w0 = w6
> 24: (95) exit
>
> Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> ---
Looks good, thanks!
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
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