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Message-ID: <CAKH8qBu3EVP7EBCO56SemGM-jR6ZMnggidEO16teLqFDDqTStg@mail.gmail.com>
Date: Wed, 13 Jan 2021 11:08:30 -0800
From: Stanislav Fomichev <sdf@...gle.com>
To: Martin KaFai Lau <kafai@...com>
Cc: Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Song Liu <songliubraving@...com>
Subject: Re: [PATCH bpf-next v7 3/4] bpf: try to avoid kzalloc in cgroup/{s,g}etsockopt
On Wed, Jan 13, 2021 at 11:03 AM Martin KaFai Lau <kafai@...com> wrote:
>
> On Tue, Jan 12, 2021 at 02:38:46PM -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@...gle.com>
> > Cc: Martin KaFai Lau <kafai@...com>
> > Cc: Song Liu <songliubraving@...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 416e7738981b..dbeef7afbbf9 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;
> >
> > @@ -1390,14 +1410,31 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
> > */
> > if (ctx.optlen != 0) {
> > *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);
> nit. zero-ing is unnecessary when memcpy() will be done later.
SG, will switch to kmalloc, thanks!
> Acked-by: Martin KaFai Lau <kafai@...com>
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