On 01/06, Martin KaFai Lau wrote:

On Tue, Jan 05, 2021 at 01:43:50PM -0800, Stanislav Fomichev wrote:

quoted

Add custom implementation of getsockopt hook for TCP_ZEROCOPY_RECEIVE.
We skip generic hooks for TCP_ZEROCOPY_RECEIVE and have a custom
call in do_tcp_getsockopt using the on-stack data. This removes
3% overhead for locking/unlocking the socket.

Also:
- Removed BUILD_BUG_ON (zerocopy doesn't depend on the buf size anymore)
- Separated on-stack buffer into bpf_sockopt_buf and downsized to 32  

bytes

quoted

  (let's keep it to help with the other options)

(I can probably split this patch into two: add new features and rework
 bpf_sockopt_buf; can follow up if the approach in general sounds
 good).

Without this patch:
     3.29%     0.07%  tcp_mmap  [kernel.kallsyms]  [k]  

__cgroup_bpf_run_filter_getsockopt

quoted

            |
             --3.22%--__cgroup_bpf_run_filter_getsockopt
                       |
                       |--0.66%--lock_sock_nested

A general question for sockopt prog, why the BPF_CGROUP_(GET|SET)SOCKOPT  
prog
has to run under lock_sock()?

I don't think there is a strong reason. We expose sk to the BPF program,
but mainly for the socket storage map (which, afaik, doesn't require
socket to be locked). OTOH, it seems that providing a consistent view
of the sk to the BPF is a good idea.

Eric has suggested to try to use fast socket lock. It helps a bit,
but it doesn't remove the issue completely because
we do a bunch of copy_{to,from}_user in the generic
__cgroup_bpf_run_filter_getsockopt as well :-(

quoted

                       |
                       |--0.57%--__might_fault
                       |
                        --0.56%--release_sock

With the patch applied:
     0.42%     0.10%  tcp_mmap  [kernel.kallsyms]  [k]  

__cgroup_bpf_run_filter_getsockopt_kern

quoted

     0.02%     0.02%  tcp_mmap  [kernel.kallsyms]  [k]  

__cgroup_bpf_run_filter_getsockopt

quoted

[ ... ]

quoted

@@ -1445,15 +1442,29 @@ int __cgroup_bpf_run_filter_getsockopt(struct  

sock *sk, int level,

quoted

 				       int __user *optlen, int max_optlen,
 				       int retval)
 {
-	struct cgroup *cgrp = sock_cgroup_ptr(&sk->sk_cgrp_data);
-	struct bpf_sockopt_kern ctx = {
-		.sk = sk,
-		.level = level,
-		.optname = optname,
-		.retval = retval,
-	};
+	struct bpf_sockopt_kern ctx;
+	struct bpf_sockopt_buf buf;
+	struct cgroup *cgrp;
 	int ret;

+#ifdef CONFIG_INET
+	/* TCP do_tcp_getsockopt has optimized getsockopt implementation
+	 * to avoid extra socket lock for TCP_ZEROCOPY_RECEIVE.
+	 */
+	if (sk->sk_prot->getsockopt == tcp_getsockopt &&
+	    level == SOL_TCP && optname == TCP_ZEROCOPY_RECEIVE)
+		return retval;
+#endif

That seems too much protocol details and not very scalable.
It is not very related to kernel/bpf/cgroup.c which has very little idea
whether a specific protocol has optimized things in some ways (e.g. by
directly calling cgroup's bpf prog at some strategic places in this  
patch).
Lets see if it can be done better.

At least, these protocol checks belong to the net's socket.c
more than the bpf's cgroup.c here.  If it also looks like layering
breakage in socket.c, may be adding a signal in sk_prot (for example)
to tell if the sk_prot->getsockopt has already called the cgroup's bpf
prog?  (e.g. tcp_getsockopt() can directly call the cgroup's bpf for all
optname instead of only TCP_ZEROCOPY_RECEIVE).

For example:

int __sys_getsockopt(...)
{
	/* ... */

	if (!sk_prot->bpf_getsockopt_handled)
		BPF_CGROUP_RUN_PROG_GETSOCKOPT(...);
}

Thoughts?

Sounds good. I didn't go that far because I don't expect there to be
a lot of special cases like that. But it might be worth supporting
it in a generic way from the beginning.

I was thinking about something simpler:

int __cgroup_bpf_run_filter_getsockopt(sk, ...)
{
	if (sk->sk_prot->bypass_bpf_getsockopt(level, optlen)) {
		return retval;
	}

  	// ...
}

Not sure it's worth exposing it to the __sys_getsockopt. WDYT?