On Wed, Feb 28, 2024 at 9:24 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Sun, Feb 25, 2024 at 2:07 AM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
> >
> > Add three new kfuncs for the bits iterator:
> > - bpf_iter_bits_new
> > Initialize a new bits iterator for a given memory area. Due to the
> > limitation of bpf memalloc, the max number of bits that can be iterated
> > over is limited to (4096 * 8).
> > - bpf_iter_bits_next
> > Get the next bit in a bpf_iter_bits
> > - bpf_iter_bits_destroy
> > Destroy a bpf_iter_bits
> >
> > The bits iterator facilitates the iteration of the bits of a memory area,
> > such as cpumask. It can be used in any context and on any address.
> >
> > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> > ---
> > kernel/bpf/helpers.c | 100 +++++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 100 insertions(+)
> >
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index 93edf730d288..052f63891834 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -2542,6 +2542,103 @@ __bpf_kfunc void bpf_throw(u64 cookie)
> > WARN(1, "A call to BPF exception callback should never return\n");
> > }
> >
> > +struct bpf_iter_bits {
> > + __u64 __opaque[2];
> > +} __aligned(8);
> > +
> > +struct bpf_iter_bits_kern {
> > + unsigned long *bits;
> > + u32 nr_bits;
> > + int bit;
> > +} __aligned(8);
> > +
> > +/**
> > + * bpf_iter_bits_new() - Initialize a new bits iterator for a given memory area
> > + * @it: The new bpf_iter_bits to be created
> > + * @unsafe_ptr__ign: A ponter pointing to a memory area to be iterated over
> > + * @nr_bits: The number of bits to be iterated over. Due to the limitation of
> > + * memalloc, it can't greater than (4096 * 8).
> > + *
> > + * This function initializes a new bpf_iter_bits structure for iterating over
> > + * a memory area which is specified by the @unsafe_ptr__ign and @nr_bits. It
> > + * copy the data of the memory area to the newly created bpf_iter_bits @it for
> > + * subsequent iteration operations.
> > + *
> > + * On success, 0 is returned. On failure, ERR is returned.
> > + */
> > +__bpf_kfunc int
> > +bpf_iter_bits_new(struct bpf_iter_bits *it, const void *unsafe_ptr__ign, u32 nr_bits)
> > +{
> > + struct bpf_iter_bits_kern *kit = (void *)it;
> > + u32 size = BITS_TO_BYTES(nr_bits);
> > + int err;
> > +
> > + BUILD_BUG_ON(sizeof(struct bpf_iter_bits_kern) != sizeof(struct bpf_iter_bits));
> > + BUILD_BUG_ON(__alignof__(struct bpf_iter_bits_kern) !=
> > + __alignof__(struct bpf_iter_bits));
> > +
> > + if (!unsafe_ptr__ign || !nr_bits) {
> > + kit->bits = NULL;
> > + return -EINVAL;
> > + }
> > +
> > + kit->bits = bpf_mem_alloc(&bpf_global_ma, size);
> > + if (!kit->bits)
> > + return -ENOMEM;
>
> it's probably going to be a pretty common case to do bits iteration
> for nr_bits<=64, right?
It's highly unlikely.
Consider the CPU count as an example; There are 256 CPUs on our AMD
EPYC servers.
> So as an optimization, instead of doing
> bpf_mem_alloc() for this case, you can just copy up to 8 bytes and
> store it in a union of `unsigned long *bits` and `unsigned long
> bits_copy`. As a performance optimization (and to reduce dependency on
> memory allocation). WDYT?
>
--
Regards
Yafang
