Subject: Re: [PATCH bpf-next v3 3/3] bpf: remove extra lock_sock for TCP_ZEROCOPY_RECEIVE

From: [email-masked]
Date: Wed, 6 Jan 2021 14:45:56 -0800

On 01/06, Martin KaFai Lau wrote:
> On Tue, Jan 05, 2021 at 01:43:50PM -0800, Stanislav Fomichev wrote:
> > 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
> >   (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
> >             |
> >              --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 :-(

> >                        |
> >                        |--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
> >      0.02%     0.02%  tcp_mmap  [kernel.kallsyms]  [k]  
> __cgroup_bpf_run_filter_getsockopt
> >
> [ ... ]

> > @@ -1445,15 +1442,29 @@ int __cgroup_bpf_run_filter_getsockopt(struct  
> sock *sk, int level,
> >  				       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?