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Date:   Thu, 31 Dec 2020 12:18:43 -0800
From:   sdf@...gle.com
To:     Martin KaFai Lau <kafai@...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 12/30, Martin KaFai Lau wrote:
> 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.
I don't see any practical use-case for TCP_ZEROCOPY_RECEIVE right now
(but you never know, maybe somebody would like to count the number
of ZQ calls? inspect the arguments? idk).

Ideally, we should bypass BPF if (optname == TCP_ZEROCOPY_RECEIVE),
but then it's not 'generic' anymore :-/

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