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 14:15:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        ast@...nel.org, daniel@...earbox.net, kernel-team@...com,
        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 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?

>         /*
>          * 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 ?

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.

> +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);
> +	}
> +}