On Fri, Jan 08, 2021 at 01:02:22PM -0800, Stanislav Fomichev wrote:
> When we attach a bpf program to cgroup/getsockopt any other getsockopt()
> syscall starts incurring kzalloc/kfree cost.
>
> Let add a small buffer on the stack and use it for small (majority)
> {s,g}etsockopt values. The buffer is small enough to fit into
> the cache line and cover the majority of simple options (most
> of them are 4 byte ints).
>
> It seems natural to do the same for setsockopt, but it's a bit more
> involved when the BPF program modifies the data (where we have to
> kmalloc). The assumption is that for the majority of setsockopt
> calls (which are doing pure BPF options or apply policy) this
> will bring some benefit as well.
>
> Without this patch (we remove about 1% __kmalloc):
> 3.38% 0.07% tcp_mmap [kernel.kallsyms] [k] __cgroup_bpf_run_filter_getsockopt
> |
> --3.30%--__cgroup_bpf_run_filter_getsockopt
> |
> --0.81%--__kmalloc
>
> Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
> Cc: Martin KaFai Lau <kafai@xxxxxx>
> Cc: Song Liu <songliubraving@xxxxxx>
> ---
> include/linux/filter.h | 5 ++++
> kernel/bpf/cgroup.c | 52 ++++++++++++++++++++++++++++++++++++------
> 2 files changed, 50 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 29c27656165b..8739f1d4cac4 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -1281,6 +1281,11 @@ struct bpf_sysctl_kern {
> u64 tmp_reg;
> };
>
> +#define BPF_SOCKOPT_KERN_BUF_SIZE 32
> +struct bpf_sockopt_buf {
> + u8 data[BPF_SOCKOPT_KERN_BUF_SIZE];
> +};
> +
> struct bpf_sockopt_kern {
> struct sock *sk;
> u8 *optval;
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index c41bb2f34013..a9aad9c419e1 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -1298,7 +1298,8 @@ static bool __cgroup_bpf_prog_array_is_empty(struct cgroup *cgrp,
> return empty;
> }
>
> -static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int max_optlen)
> +static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int max_optlen,
> + struct bpf_sockopt_buf *buf)
> {
> if (unlikely(max_optlen < 0))
> return -EINVAL;
> @@ -1310,6 +1311,15 @@ static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int max_optlen)
> max_optlen = PAGE_SIZE;
> }
>
> + if (max_optlen <= sizeof(buf->data)) {
> + /* When the optval fits into BPF_SOCKOPT_KERN_BUF_SIZE
> + * bytes avoid the cost of kzalloc.
> + */
> + ctx->optval = buf->data;
> + ctx->optval_end = ctx->optval + max_optlen;
> + return max_optlen;
> + }
> +
> ctx->optval = kzalloc(max_optlen, GFP_USER);
> if (!ctx->optval)
> return -ENOMEM;
> @@ -1319,16 +1329,26 @@ static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int max_optlen)
> return max_optlen;
> }
>
> -static void sockopt_free_buf(struct bpf_sockopt_kern *ctx)
> +static void sockopt_free_buf(struct bpf_sockopt_kern *ctx,
> + struct bpf_sockopt_buf *buf)
> {
> + if (ctx->optval == buf->data)
> + return;
> kfree(ctx->optval);
> }
>
> +static bool sockopt_buf_allocated(struct bpf_sockopt_kern *ctx,
> + struct bpf_sockopt_buf *buf)
> +{
> + return ctx->optval != buf->data;
> +}
> +
> int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
> int *optname, char __user *optval,
> int *optlen, char **kernel_optval)
> {
> struct cgroup *cgrp = sock_cgroup_ptr(&sk->sk_cgrp_data);
> + struct bpf_sockopt_buf buf = {};
> struct bpf_sockopt_kern ctx = {
> .sk = sk,
> .level = *level,
> @@ -1350,7 +1370,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
> */
> max_optlen = max_t(int, 16, *optlen);
>
> - max_optlen = sockopt_alloc_buf(&ctx, max_optlen);
> + max_optlen = sockopt_alloc_buf(&ctx, max_optlen, &buf);
> if (max_optlen < 0)
> return max_optlen;
>
> @@ -1390,13 +1410,30 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
> */
> if (ctx.optlen != 0) {
When ctx.optlen == 0, is sockopt_free_buf() called?
Did I miss something?
> *optlen = ctx.optlen;
> - *kernel_optval = ctx.optval;
> + /* We've used bpf_sockopt_kern->buf as an intermediary
> + * storage, but the BPF program indicates that we need
> + * to pass this data to the kernel setsockopt handler.
> + * No way to export on-stack buf, have to allocate a
> + * new buffer.
> + */
> + if (!sockopt_buf_allocated(&ctx, &buf)) {
> + void *p = kzalloc(ctx.optlen, GFP_USER);
> +
> + if (!p) {
> + ret = -ENOMEM;
> + goto out;
> + }
> + memcpy(p, ctx.optval, ctx.optlen);
> + *kernel_optval = p;
> + } else {
> + *kernel_optval = ctx.optval;
> + }
> }
> }
>
> out:
> if (ret)
> - sockopt_free_buf(&ctx);
> + sockopt_free_buf(&ctx, &buf);
> return ret;
> }
>
