Re: [PATCH bpf-next v2 1/3] bpf: Introduce BPF_F_CPU flag for percpu_array maps

["Leon Hwang" <leon.hwang@linux.dev>]

On Fri Aug 8, 2025 at 1:20 AM +08, Alexei Starovoitov wrote:
> On Tue, Aug 5, 2025 at 9:30 AM Leon Hwang <leon.hwang@linux.dev> wrote:
>>
>> Introduce support for the BPF_F_CPU flag in percpu_array maps to allow
>> updating values for specified CPU or for all CPUs with a single value.
>>
>> This enhancement enables:
>>
>> * Efficient update of all CPUs using a single value when cpu == (u32)~0.
>> * Targeted update or lookup for a specified CPU otherwise.
>>
>> The flag is passed via:
>>
>> * map_flags in bpf_percpu_array_update() along with embedded cpu field.
>> * elem_flags in generic_map_update_batch() along with embedded cpu field.
>>
>> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
>> ---
>>  include/linux/bpf.h            |  3 +-
>>  include/uapi/linux/bpf.h       |  6 +++
>>  kernel/bpf/arraymap.c          | 54 ++++++++++++++++++------
>>  kernel/bpf/syscall.c           | 77 +++++++++++++++++++++-------------
>>  tools/include/uapi/linux/bpf.h |  6 +++
>>  5 files changed, 103 insertions(+), 43 deletions(-)
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index cc700925b802f..c17c45f797ed9 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -2691,7 +2691,8 @@ int map_set_for_each_callback_args(struct bpf_verifier_env *env,
>>                                    struct bpf_func_state *callee);
>>
>>  int bpf_percpu_hash_copy(struct bpf_map *map, void *key, void *value);
>> -int bpf_percpu_array_copy(struct bpf_map *map, void *key, void *value);
>> +int bpf_percpu_array_copy(struct bpf_map *map, void *key, void *value,
>> +                         u64 flags);
>>  int bpf_percpu_hash_update(struct bpf_map *map, void *key, void *value,
>>                            u64 flags);
>>  int bpf_percpu_array_update(struct bpf_map *map, void *key, void *value,
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index 233de8677382e..67bc35e4d6a8d 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -1372,6 +1372,12 @@ enum {
>>         BPF_NOEXIST     = 1, /* create new element if it didn't exist */
>>         BPF_EXIST       = 2, /* update existing element */
>>         BPF_F_LOCK      = 4, /* spin_lock-ed map_lookup/map_update */
>> +       BPF_F_CPU       = 8, /* map_update for percpu_array */
>
> only percpu_array?!
> Aren't you doing it for percpu_hash too?
>

Only percpu_array in this patchset.

I have no need to do it for percpu_hash.

> The comment should also say that upper 32-bit of flags is a cpu number.
>
>> +};
>> +
>> +enum {
>> +       /* indicate updating value across all CPUs for percpu maps. */
>> +       BPF_ALL_CPUS    = (__u32)~0,
>>  };
>
> The name is inconsistent with BPF_F_ that was adopted long ago.
>
> Also looking at the implementation that ~0 looks too magical.
> imo it's cleaner to add another BPF_F_ALL_CPUS flag.
> BPF_F_CPU = 8 and upper 32-bit select a cpu.
> BPF_F_ALL_CPUS = 16 -> all cpus.

Sure, let us add these two flags:

       BPF_F_CPU       = 8, /* cpu flag for percpu maps, upper 32-bit of flags is a cpu number */
       BPF_F_ALL_CPUS  = 16, /* update value across all CPUs for percpu maps */

Thanks,
Leon