linux-kernel - Re: [PATCH v3 perf, bpf-next 1/4] perf, bpf: Introduce PERF_RECORD_BPF

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Date:   Wed, 12 Dec 2018 17:09:17 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     lkml <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "acme@...nel.org" <acme@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v3 perf, bpf-next 1/4] perf, bpf: Introduce
 PERF_RECORD_BPF_EVENT



> On Dec 12, 2018, at 5:15 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Tue, Dec 11, 2018 at 03:33:47PM -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.
>> 
>> Each BPF program may contain up to BPF_MAX_SUBPROGS (256) sub programs.
>> The following example shows kernel symbols for a BPF program with 7
>> sub programs:
>> 
>>    ffffffffa0257cf9 t bpf_prog_b07ccb89267cf242_F
>>    ffffffffa02592e1 t bpf_prog_2dcecc18072623fc_F
>>    ffffffffa025b0e9 t bpf_prog_bb7a405ebaec5d5c_F
>>    ffffffffa025dd2c t bpf_prog_a7540d4a39ec1fc7_F
>>    ffffffffa025fcca t bpf_prog_05762d4ade0e3737_F
>>    ffffffffa026108f t bpf_prog_db4bd11e35df90d4_F
>>    ffffffffa0263f00 t bpf_prog_89d64e4abf0f0126_F
>>    ffffffffa0257cf9 t bpf_prog_ae31629322c4b018__dummy_tracepoi
> 
> Doesn't BPF have enough information to generate 'saner' names? Going by
> the thing below, these sub-progs are actually functions, right?

These sub programs/functions will have their descriptive names from BTF
function types (coming in 4.21). However, BTF is optional in normal cases, 
when BTF is missing, they will be named as bpf_prog_<tag>_F. The main BPF
program has a name up to 16 byte long. In the example above, the last 
program has name _dummy_tracepoint. 

I think these sub programs are more like "programs" than "functions", 
because each sub program occupies its own page(s). 

> 
>>        /*
>>         * Record different types of bpf events:
>>         *  enum perf_bpf_event_type {
>>         *     PERF_BPF_EVENT_UNKNOWN           = 0,
>>         *     PERF_BPF_EVENT_PROG_LOAD         = 1,
>>         *     PERF_BPF_EVENT_PROG_UNLOAD       = 2,
>>         *  };
>>         *
>>         * struct {
>>         *      struct perf_event_header header;
>>         *      u32                             type;
>>         *      u32                             flags;
>>         *      u32                             id; // prog_id or other id
>>         *      u32                             sub_id; // subprog id
>>         *
>>         *      // for bpf_prog types, bpf prog or subprog
>>         *      u8                              tag[BPF_TAG_SIZE];
>>         *      u64                             addr;
>>         *      u64                             len;
>>         *      char                            name[];
>>         *      struct sample_id                sample_id;
>>         * };
>>         */
> 
> Isn't this mixing two different things (poorly)? The kallsym update and
> the BPF load/unload ?

I would say these two things are actually two parts of the same event. 
Fields id, sub_id, and tag provide information about which program is
mapped to this ksym. They are equivalent to "pgoff + filename" in 
PERF_RECORD_MMAP, or "maj, min, ino, and ino_generation" in 
PERF_RECORD_MMAP2. 

> 
> And while this tracks the bpf kallsyms, it does not do all kallsyms.
> 
> .... Oooh, I see the problem, everybody is doing their own custom
> kallsym_{add,del}() thing, instead of having that in generic code :-(
> 
> This, for example, doesn't track module load/unload nor ftrace
> trampolines, even though both affect kallsyms.

I think we can use PERF_RECORD_MMAP(or MMAP2) for module load/unload. 
That could be separate sets of patches. 

Thanks,
Song

> 
>> +void perf_event_bpf_event_prog(enum perf_bpf_event_type type,
>> +			       struct bpf_prog *prog)
>> +{
>> +	if (!atomic_read(&nr_bpf_events))
>> +		return;
>> +
>> +	if (type != PERF_BPF_EVENT_PROG_LOAD &&
>> +	    type != PERF_BPF_EVENT_PROG_UNLOAD)
>> +		return;
>> +
>> +	if (prog->aux->func_cnt == 0) {
>> +		perf_event_bpf_event_subprog(type, prog,
>> +					     prog->aux->id, 0);
>> +	} else {
>> +		int i;
>> +
>> +		for (i = 0; i < prog->aux->func_cnt; i++)
>> +			perf_event_bpf_event_subprog(type, prog->aux->func[i],
>> +						     prog->aux->id, i);
>> +	}
>> +}
> 
>