From: Leon Hwang <leon.hwang@linux.dev>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: bpf@vger.kernel.org, ast@kernel.org, andrii@kernel.org,
daniel@iogearbox.net
Subject: Re: [RFC PATCH bpf-next v2 1/3] bpf: Introduce BPF_F_CPU flag for percpu_array map
Date: Mon, 14 Jul 2025 20:47:04 +0800 [thread overview]
Message-ID: <592129b2-0442-4b10-8b56-0e15d2ce113e@linux.dev> (raw)
In-Reply-To: <CAEf4BzZCzd0VGNBoLOd=ENxPnwsynuwvKdNYkKhUc7ARFCudSQ@mail.gmail.com>
On 2025/7/12 02:10, Andrii Nakryiko wrote:
> On Mon, Jul 7, 2025 at 9:04 AM Leon Hwang <leon.hwang@linux.dev> wrote:
>>
>> This patch introduces support for the BPF_F_CPU flag in percpu_array maps
>> to allow updating or looking up values for specified CPUs or for all CPUs
>> with a single value.
>>
>
> For next revision, please drop RFC tag, so this is tested and reviewed
> as a proper patch set.
>
Sure. I'll drop it.
>> 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 separated cpu field.
>>
>> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
>> ---
>> include/linux/bpf.h | 3 +-
>> include/uapi/linux/bpf.h | 7 +++++
>> kernel/bpf/arraymap.c | 56 ++++++++++++++++++++++++++--------
>> kernel/bpf/syscall.c | 52 +++++++++++++++++++------------
>> tools/include/uapi/linux/bpf.h | 7 +++++
>> 5 files changed, 92 insertions(+), 33 deletions(-)
>>
>
> [...]
>
>>
>> + cpu = (u32)(flags >> 32);
>> + flags &= (u32)~0;
>> + if (unlikely(flags > BPF_F_CPU))
>> + return -EINVAL;
>> + if (unlikely((flags & BPF_F_CPU) && cpu >= num_possible_cpus()))
>> + return -E2BIG;
>> +
>> /* per_cpu areas are zero-filled and bpf programs can only
>> * access 'value_size' of them, so copying rounded areas
>> * will not leak any kernel data
>> @@ -313,10 +320,15 @@ int bpf_percpu_array_copy(struct bpf_map *map, void *key, void *value)
>> size = array->elem_size;
>> rcu_read_lock();
>> pptr = array->pptrs[index & array->index_mask];
>> - for_each_possible_cpu(cpu) {
>> - copy_map_value_long(map, value + off, per_cpu_ptr(pptr, cpu));
>> - check_and_init_map_value(map, value + off);
>> - off += size;
>> + if (flags & BPF_F_CPU) {
>> + copy_map_value_long(map, value, per_cpu_ptr(pptr, cpu));
>> + check_and_init_map_value(map, value);
>> + } else {
>> + for_each_possible_cpu(cpu) {
>> + copy_map_value_long(map, value + off, per_cpu_ptr(pptr, cpu));
>> + check_and_init_map_value(map, value + off);
>> + off += size;
>> + }
>> }
>> rcu_read_unlock();
>> return 0;
>> @@ -387,13 +399,21 @@ int bpf_percpu_array_update(struct bpf_map *map, void *key, void *value,
>> struct bpf_array *array = container_of(map, struct bpf_array, map);
>> u32 index = *(u32 *)key;
>> void __percpu *pptr;
>> - int cpu, off = 0;
>> - u32 size;
>> + bool reuse_value;
>> + u32 size, cpu;
>> + int off = 0;
>>
>> - if (unlikely(map_flags > BPF_EXIST))
>> + cpu = (u32)(map_flags >> 32);
>> + map_flags = map_flags & (u32)~0;
>
> be consistent, use &= approach as above
>
Ack.
>> + if (unlikely(map_flags > BPF_F_CPU))
>> /* unknown flags */
>> return -EINVAL;
>>
>> + if (unlikely((map_flags & BPF_F_CPU) && cpu != BPF_ALL_CPUS &&
>> + cpu >= num_possible_cpus()))
>> + /* invalid cpu */
>> + return -E2BIG;
>> +
>> if (unlikely(index >= array->map.max_entries))
>> /* all elements were pre-allocated, cannot insert a new one */
>> return -E2BIG;
>> @@ -409,12 +429,22 @@ int bpf_percpu_array_update(struct bpf_map *map, void *key, void *value,
>> * so no kernel data leaks possible
>> */
>> size = array->elem_size;
>> + reuse_value = (map_flags & BPF_F_CPU) && cpu == BPF_ALL_CPUS;
>
> I find "reuse_value" name extremely misleading, I stumble upon this
> every time (because "value" is ambiguous, is it the source value or
> map value we are updating?). Please drop it, there is no need for it,
> just do `map_flags & BPF_F_CPU` check in that for_each_possible_cpu
> loop below
>
Ack.
>> rcu_read_lock();
>> pptr = array->pptrs[index & array->index_mask];
>> - for_each_possible_cpu(cpu) {
>> - copy_map_value_long(map, per_cpu_ptr(pptr, cpu), value + off);
>> + if ((map_flags & BPF_F_CPU) && cpu != BPF_ALL_CPUS) {
>> + copy_map_value_long(map, per_cpu_ptr(pptr, cpu), value);
>> bpf_obj_free_fields(array->map.record, per_cpu_ptr(pptr, cpu));
>> - off += size;
>> + } else {
>> + for_each_possible_cpu(cpu) {
>> + if (!reuse_value) {
>> + copy_map_value_long(map, per_cpu_ptr(pptr, cpu), value + off);
>> + off += size;
>> + } else {
>> + copy_map_value_long(map, per_cpu_ptr(pptr, cpu), value);
>> + }
>
> simpler and less duplication:
>
> copy_map_value_long(map, per_cpu_ptr(pptr, cpu), value + off);
> /*
> * same user-provided value is used if BPF_F_CPU is specified,
> * otherwise value is an array of per-cpu values
> */
> if (!(map_flags & BPF_F_CPU))
> off += size;
>
LGTM.
>> + bpf_obj_free_fields(array->map.record, per_cpu_ptr(pptr, cpu));
>> + }
>> }
>> rcu_read_unlock();
>> return 0;
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index 7db7182a3057..a3ce0cdecb3c 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -129,8 +129,12 @@ bool bpf_map_write_active(const struct bpf_map *map)
>> return atomic64_read(&map->writecnt) != 0;
>> }
>>
>> -static u32 bpf_map_value_size(const struct bpf_map *map)
>> +static u32 bpf_map_value_size(const struct bpf_map *map, u64 flags)
>> {
>> + if ((flags & BPF_F_CPU) &&
>> + map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY)
>
> formatting is off, keep single line
>
Ack.
>> + return round_up(map->value_size, 8);
>> +
>> if (map->map_type == BPF_MAP_TYPE_PERCPU_HASH ||
>> map->map_type == BPF_MAP_TYPE_LRU_PERCPU_HASH ||
>> map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY ||
>> @@ -312,7 +316,7 @@ static int bpf_map_copy_value(struct bpf_map *map, void *key, void *value,
>> map->map_type == BPF_MAP_TYPE_LRU_PERCPU_HASH) {
>> err = bpf_percpu_hash_copy(map, key, value);
>> } else if (map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY) {
>> - err = bpf_percpu_array_copy(map, key, value);
>> + err = bpf_percpu_array_copy(map, key, value, flags);
>> } else if (map->map_type == BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE) {
>> err = bpf_percpu_cgroup_storage_copy(map, key, value);
>> } else if (map->map_type == BPF_MAP_TYPE_STACK_TRACE) {
>> @@ -1662,7 +1666,7 @@ static int map_lookup_elem(union bpf_attr *attr)
>> if (CHECK_ATTR(BPF_MAP_LOOKUP_ELEM))
>> return -EINVAL;
>>
>> - if (attr->flags & ~BPF_F_LOCK)
>> + if ((attr->flags & (u32)~0) & ~(BPF_F_LOCK | BPF_F_CPU))
>
> nit: this whole `attr->flags & (u32)~0` looks like an over-engineered
> `(u32)attr->flags`...
>
>> return -EINVAL;
>
> we should probably also have a condition checking that upper 32 bits
> are zero if BPF_F_CPU is not set?
>
Correct. We should check the upper 32 bits if BPF_F_CPU is not set. It
should check the flags like
`(attr->flags & ~BPF_F_LOCK) && !(attr->flags & BPF_F_CPU)`.
>>
>> CLASS(fd, f)(attr->map_fd);
>> @@ -1680,7 +1684,7 @@ static int map_lookup_elem(union bpf_attr *attr)
>> if (IS_ERR(key))
>> return PTR_ERR(key);
>>
>> - value_size = bpf_map_value_size(map);
>> + value_size = bpf_map_value_size(map, attr->flags);
>>
>> err = -ENOMEM;
>> value = kvmalloc(value_size, GFP_USER | __GFP_NOWARN);
>> @@ -1749,7 +1753,7 @@ static int map_update_elem(union bpf_attr *attr, bpfptr_t uattr)
>> goto err_put;
>> }
>>
>> - value_size = bpf_map_value_size(map);
>> + value_size = bpf_map_value_size(map, attr->flags);
>> value = kvmemdup_bpfptr(uvalue, value_size);
>> if (IS_ERR(value)) {
>> err = PTR_ERR(value);
>> @@ -1941,19 +1945,25 @@ int generic_map_update_batch(struct bpf_map *map, struct file *map_file,
>> {
>> void __user *values = u64_to_user_ptr(attr->batch.values);
>> void __user *keys = u64_to_user_ptr(attr->batch.keys);
>> - u32 value_size, cp, max_count;
>> + u32 value_size, cp, max_count, cpu = attr->batch.cpu;
>> + u64 elem_flags = attr->batch.elem_flags;
>> void *key, *value;
>> int err = 0;
>>
>> - if (attr->batch.elem_flags & ~BPF_F_LOCK)
>> + if (elem_flags & ~(BPF_F_LOCK | BPF_F_CPU))
>> return -EINVAL;
>>
>> - if ((attr->batch.elem_flags & BPF_F_LOCK) &&
>> + if ((elem_flags & BPF_F_LOCK) &&
>> !btf_record_has_field(map->record, BPF_SPIN_LOCK)) {
>> return -EINVAL;
>> }
>>
>> - value_size = bpf_map_value_size(map);
>> + if ((elem_flags & BPF_F_CPU) &&
>> + map->map_type != BPF_MAP_TYPE_PERCPU_ARRAY)
>> + return -EINVAL;
>> +
>> + value_size = bpf_map_value_size(map, elem_flags);
>> + elem_flags = (((u64)cpu) << 32) | elem_flags;
>
> nit: elem_flags |= (u64)cpu << 32;
>
> same effect, but a bit more explicitly stating "we are just adding
> stuff to elem_flags"
>
Ack.
>>
>> max_count = attr->batch.count;
>> if (!max_count)
>> @@ -1979,8 +1989,7 @@ int generic_map_update_batch(struct bpf_map *map, struct file *map_file,
>> copy_from_user(value, values + cp * value_size, value_size))
>> break;
>>
>> - err = bpf_map_update_value(map, map_file, key, value,
>> - attr->batch.elem_flags);
>> + err = bpf_map_update_value(map, map_file, key, value, elem_flags);
>>
>> if (err)
>> break;
>> @@ -2004,18 +2013,24 @@ int generic_map_lookup_batch(struct bpf_map *map,
>> void __user *ubatch = u64_to_user_ptr(attr->batch.in_batch);
>> void __user *values = u64_to_user_ptr(attr->batch.values);
>> void __user *keys = u64_to_user_ptr(attr->batch.keys);
>> + u32 value_size, cp, max_count, cpu = attr->batch.cpu;
>> void *buf, *buf_prevkey, *prev_key, *key, *value;
>> - u32 value_size, cp, max_count;
>> + u64 elem_flags = attr->batch.elem_flags;
>> int err;
>>
>> - if (attr->batch.elem_flags & ~BPF_F_LOCK)
>> + if (elem_flags & ~(BPF_F_LOCK | BPF_F_CPU))
>> return -EINVAL;
>>
>> - if ((attr->batch.elem_flags & BPF_F_LOCK) &&
>> + if ((elem_flags & BPF_F_LOCK) &&
>> !btf_record_has_field(map->record, BPF_SPIN_LOCK))
>> return -EINVAL;
>>
>> - value_size = bpf_map_value_size(map);
>> + if ((elem_flags & BPF_F_CPU) &&
>> + map->map_type != BPF_MAP_TYPE_PERCPU_ARRAY)
>> + return -EINVAL;
>> +
>> + value_size = bpf_map_value_size(map, elem_flags);
>> + elem_flags = (((u64)cpu) << 32) | elem_flags;
>>
>
> ditto
>
Ack.
>> max_count = attr->batch.count;
>> if (!max_count)
>
> [...]
next prev parent reply other threads:[~2025-07-14 12:47 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-07-07 16:04 [RFC PATCH bpf-next v2 0/3] bpf: Introduce BPF_F_CPU flag for percpu_array map Leon Hwang
2025-07-07 16:04 ` [RFC PATCH bpf-next v2 1/3] " Leon Hwang
2025-07-11 18:10 ` Andrii Nakryiko
2025-07-14 12:47 ` Leon Hwang [this message]
2025-07-07 16:04 ` [RFC PATCH bpf-next v2 2/3] bpf, libbpf: Support BPF_F_CPU " Leon Hwang
2025-07-11 18:10 ` Andrii Nakryiko
2025-07-14 12:48 ` Leon Hwang
2025-07-07 16:04 ` [RFC PATCH bpf-next v2 3/3] selftests/bpf: Add case to test BPF_F_CPU Leon Hwang
2025-07-11 18:11 ` Andrii Nakryiko
2025-07-14 12:49 ` Leon Hwang
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