netdev - Re: [PATCH bpf-next 1/2] bpf: try to avoid kzalloc in cgroup/{s,g}etsockopt

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Message-ID: <20201231065038.k637ewwyqclq2nxh@kafai-mbp.dhcp.thefacebook.com>
Date:   Wed, 30 Dec 2020 22:50:44 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     <sdf@...gle.com>
CC:     <netdev@...r.kernel.org>, <bpf@...r.kernel.org>, <ast@...nel.org>,
        <daniel@...earbox.net>
Subject: Re: [PATCH bpf-next 1/2] bpf: try to avoid kzalloc in
 cgroup/{s,g}etsockopt

On Tue, Dec 22, 2020 at 07:09:33PM -0800, sdf@...gle.com wrote:
> On 12/22, Martin KaFai Lau wrote:
> > On Thu, Dec 17, 2020 at 09:23:23AM -0800, Stanislav Fomichev wrote:
> > > When we attach a bpf program to cgroup/getsockopt any other getsockopt()
> > > syscall starts incurring kzalloc/kfree cost. While, in general, it's
> > > not an issue, sometimes it is, like in the case of TCP_ZEROCOPY_RECEIVE.
> > > TCP_ZEROCOPY_RECEIVE (ab)uses getsockopt system call to implement
> > > fastpath for incoming TCP, we don't want to have extra allocations in
> > > there.
> > >
> > > Let add a small buffer on the stack and use it for small (majority)
> > > {s,g}etsockopt values. I've started with 128 bytes to cover
> > > the options we care about (TCP_ZEROCOPY_RECEIVE which is 32 bytes
> > > currently, with some planned extension to 64 + some headroom
> > > for the future).
> > >
> > > 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.
> > >
> > > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > > ---
> > >  include/linux/filter.h |  3 +++
> > >  kernel/bpf/cgroup.c    | 41 +++++++++++++++++++++++++++++++++++++++--
> > >  2 files changed, 42 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/linux/filter.h b/include/linux/filter.h
> > > index 29c27656165b..362eb0d7af5d 100644
> > > --- a/include/linux/filter.h
> > > +++ b/include/linux/filter.h
> > > @@ -1281,6 +1281,8 @@ struct bpf_sysctl_kern {
> > >  	u64 tmp_reg;
> > >  };
> > >
> > > +#define BPF_SOCKOPT_KERN_BUF_SIZE	128
> > Since these 128 bytes (which then needs to be zero-ed) is modeled after
> > the TCP_ZEROCOPY_RECEIVE use case, it will be useful to explain
> > a use case on how the bpf prog will interact with
> > getsockopt(TCP_ZEROCOPY_RECEIVE).
> The only thing I would expect BPF program can do is to return EPERM
> to cause application to fallback to non-zerocopy path (and, mostly,
> bypass). I don't think BPF can meaningfully mangle the data in struct
> tcp_zerocopy_receive.
> 
> Does it address your concern? Or do you want me to add a comment or
> something?
I was asking because, while 128 byte may work best for TCP_ZEROCOPY_RECEIVCE,
it is many unnecessary byte-zeroings for most options though.
Hence, I am interested to see if there is a practical bpf
use case for TCP_ZEROCOPY_RECEIVE.