netdev - Re: [PATCH bpf-next v3 2/3] libbpf: add low level TC-BPF API

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Message-ID: <8735vih4h9.fsf@toke.dk>
Date:   Thu, 22 Apr 2021 14:46:10 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 2/3] libbpf: add low level TC-BPF API

Daniel Borkmann <daniel@...earbox.net> writes:

> On 4/22/21 11:08 AM, Toke Høiland-Jørgensen wrote:
>> Daniel Borkmann <daniel@...earbox.net> writes:
>>> On 4/21/21 9:48 PM, Toke Høiland-Jørgensen wrote:
>>>> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>>>>> On Tue, Apr 20, 2021 at 12:37 PM Kumar Kartikeya Dwivedi
>>>>> <memxor@...il.com> wrote:
>>> [...]
>>>>>> ---
>>>>>>    tools/lib/bpf/libbpf.h   |  44 ++++++
>>>>>>    tools/lib/bpf/libbpf.map |   3 +
>>>>>>    tools/lib/bpf/netlink.c  | 319 ++++++++++++++++++++++++++++++++++++++-
>>>>>>    3 files changed, 360 insertions(+), 6 deletions(-)
>>>>>>
>>>>>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>>>>>> index bec4e6a6e31d..b4ed6a41ea70 100644
>>>>>> --- a/tools/lib/bpf/libbpf.h
>>>>>> +++ b/tools/lib/bpf/libbpf.h
>>>>>> @@ -16,6 +16,8 @@
>>>>>>    #include <stdbool.h>
>>>>>>    #include <sys/types.h>  // for size_t
>>>>>>    #include <linux/bpf.h>
>>>>>> +#include <linux/pkt_sched.h>
>>>>>> +#include <linux/tc_act/tc_bpf.h>
>>>>>
>>>>> apart from those unused macros below, are these needed in public API header?
>>>>>
>>>>>>    #include "libbpf_common.h"
>>>>>>
>>>>>> @@ -775,6 +777,48 @@ LIBBPF_API int bpf_linker__add_file(struct bpf_linker *linker, const char *filen
>>>>>>    LIBBPF_API int bpf_linker__finalize(struct bpf_linker *linker);
>>>>>>    LIBBPF_API void bpf_linker__free(struct bpf_linker *linker);
>>>>>>
>>>>>> +/* Convenience macros for the clsact attach hooks */
>>>>>> +#define BPF_TC_CLSACT_INGRESS TC_H_MAKE(TC_H_CLSACT, TC_H_MIN_INGRESS)
>>>>>> +#define BPF_TC_CLSACT_EGRESS TC_H_MAKE(TC_H_CLSACT, TC_H_MIN_EGRESS)
>>>>>
>>>>> these seem to be used only internally, why exposing them in public
>>>>> API?
>>>>
>>>> No they're "aliases" for when you want to attach the filter directly to
>>>> the interface (and thus install the clsact qdisc as the root). You can
>>>> also use the filter with an existing qdisc (most commonly HTB), in which
>>>> case you need to specify the qdisc handle as the root. We have a few
>>>> examples of this use case:
>>>>
>>>> https://github.com/xdp-project/bpf-examples/tree/master/traffic-pacing-edt
>>>> and
>>>> https://github.com/xdp-project/xdp-cpumap-tc
>>>
>>> I'm a bit puzzled, could you elaborate on your use case on why you wouldn't
>>> use the tc egress hook for those especially given it's guaranteed to run
>>> outside of root qdisc lock?
>> 
>> Jesper can correct me if I'm wrong, but I think the first one of the
>> links above is basically his implementation of just that EDT-based
>> shaper. And it works reasonably well, except you don't get the nice
>> per-flow scheduling and sparse flow prioritisation like in FQ-CoDel
>> unless you implement that yourself in BPF when you set the timestamps
>> (and that is by no means trivial to implement).
>> 
>> So if you want to use any of the features of the existing qdiscs (I have
>> also been suggesting to people that they use tc_bpf if they want to
>> customise sch_cake's notion of flows or shaping tiers), you need to be
>> able to attach the filter to an existing qdisc. Sure, this means you're
>> still stuck behind the qdisc lock, but for some applications that is
>> fine (not everything is a data centre, some devices don't have that many
>> CPUs anyway; and as the second example above shows, you can get around
>> the qdisc lock by some clever use of partitioning of flows using
>> cpumap).
>> 
>> So what this boils down to is, we should keep the 'parent' parameter not
>> just as an egress/ingress enum, but also as a field the user can fill
>> out. I'm fine with moving the latter into the opts struct, though, so
>> maybe the function parameter can be an enum like:
>> 
>> enum bpf_tc_attach_point {
>>    BPF_TC_CLSACT_INGRESS,
>>    BPF_TC_CLSACT_EGRESS,
>>    BPF_TC_QDISC_PARENT
>> };
>> 
>> where if you set the last one you have to fill in the parent in opts?
>
> Fair enough, I still think this is a bit backwards and should be discouraged
> given the constraints, but if you have an actual need for it ... I'd rather
> simplify API naming, the fact that it's clsact is an implementation detail
> and shouldn't matter to a user, like:
>
> enum bpf_tc_attach_point {
> 	BPF_TC_INGRESS,
> 	BPF_TC_EGRESS,
> 	BPF_TC_CUSTOM_PARENT,
> };
>
> For BPF_TC_INGRESS and BPF_TC_EGRESS, I would enforce opts parent parameter
> to be /zero/ from the API.

OK, SGTM :)

-Toke