Re: [PATCH bpf-next v6 2/3] bpf: try to avoid kzalloc in cgroup/{s,g}etsockopt

[Martin KaFai Lau <kafai@fb.com>]

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@google.com>
> Cc: Martin KaFai Lau <kafai@fb.com>
> Cc: Song Liu <songliubraving@fb.com>
> ---
>  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;
>  }
>