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From:   Song Liu <songliubraving@fb.com>
To:     Peter Zijlstra <peterz@infradead.org>
CC:     Netdev <netdev@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        "ast@kernel.org" <ast@kernel.org>,
        "daniel@iogearbox.net" <daniel@iogearbox.net>,
        "acme@kernel.org" <acme@kernel.org>
Subject: Re: [RFC perf,bpf 1/5] perf, bpf: Introduce PERF_RECORD_BPF_EVENT
Thread-Topic: [RFC perf,bpf 1/5] perf, bpf: Introduce PERF_RECORD_BPF_EVENT
Thread-Index: AQHUdhLRNu0EDS9AC0KZ+9MuHYh4daVD/w6AgACjMoA=
Date:   Wed, 7 Nov 2018 18:25:04 +0000
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> On Nov 7, 2018, at 12:40 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
> On Tue, Nov 06, 2018 at 12:52:42PM -0800, Song Liu wrote:
>> For better performance analysis of BPF programs, this patch introduces
>> PERF_RECORD_BPF_EVENT, a new perf_event_type that exposes BPF program
>> load/unload information to user space.
>>=20
>>        /*
>>         * Record different types of bpf events:
>>         *   enum perf_bpf_event_type {
>>         *      PERF_BPF_EVENT_UNKNOWN          =3D 0,
>>         *      PERF_BPF_EVENT_PROG_LOAD        =3D 1,
>>         *      PERF_BPF_EVENT_PROG_UNLOAD      =3D 2,
>>         *   };
>>         *
>>         * struct {
>>         *      struct perf_event_header header;
>>         *      u16 type;
>>         *      u16 flags;
>>         *      u32 id;  // prog_id or map_id
>>         * };
>>         */
>>        PERF_RECORD_BPF_EVENT                   =3D 17,
>>=20
>> PERF_RECORD_BPF_EVENT contains minimal information about the BPF program=
.
>> Perf utility (or other user space tools) should listen to this event and
>> fetch more details about the event via BPF syscalls
>> (BPF_PROG_GET_FD_BY_ID, BPF_OBJ_GET_INFO_BY_FD, etc.).
>=20
> Why !? You're failing to explain why it cannot provide the full
> information there.

Aha, I missed this part. I will add the following to next version. Please
let me know if anything is not clear.

Thanks,
Song


This design decision is picked for the following reasons. First, BPF=20
programs could be loaded-and-jited and/or unloaded before/during/after=20
perf-record run. Once a BPF programs is unloaded, it is impossible to=20
recover details of the program. It is impossible to provide the=20
information through a simple key (like the build ID). Second, BPF prog
annotation is under fast developments. Multiple informations will be=20
added to bpf_prog_info in the next few releases. Including all the
information of a BPF program in the perf ring buffer requires frequent=20
changes to the perf ABI, and thus makes it very difficult to manage=20
compatibility of perf utility.=20

For more information on this topic, please refer to the following=20
discussions:=20

    https://www.spinics.net/lists/netdev/msg524230.html