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

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Date:   Fri, 14 Dec 2018 17:10:56 +0000
From:   Song Liu <songliubraving@...com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        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>
Subject: Re: [PATCH v3 perf, bpf-next 1/4] perf, bpf: Introduce
 PERF_RECORD_BPF_EVENT



> On Dec 14, 2018, at 5:48 AM, Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> 
> Em Thu, Dec 13, 2018 at 09:48:57PM +0000, Song Liu escreveu:
>> 
>> 
>>> On Dec 13, 2018, at 10:45 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>> 
>>> On Wed, Dec 12, 2018 at 01:33:20PM -0500, Steven Rostedt wrote:
>>>> On Wed, 12 Dec 2018 19:05:53 +0100
>>>> Peter Zijlstra <peterz@...radead.org> wrote:
>>>> 
>>>>> On Wed, Dec 12, 2018 at 05:09:17PM +0000, Song Liu wrote:
>>>>>>> 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.   
>>>>> 
>>>>> So I would actually like to move bpf_lock/bpf_kallsyms/bpf_tree +
>>>>> bpf_prog_kallsyms_*() + __bpf_address_lookup() into kernel/kallsyms.c
>>>>> and also have ftrace use that.
>>>>> 
>>>>> Because currently the ftrace stuff is otherwise invisible.
>>>>> 
>>>>> A generic kallsym register/unregister for any JIT.
>>>> 
>>>> That's if it needs to look up the symbols that were recorded when init
>>>> was unloaded.
>>>> 
>>>> The ftrace kallsyms is used to save the function names of init code
>>>> that was freed, but may have been recorded. With out the ftrace
>>>> kallsyms the functions traced at init time would just show up as hex
>>>> addresses (not very useful).
>>>> 
>>>> I'm not sure how BPF would need those symbols unless they were executed
>>>> during init (module or core) and needed to see what the symbols use to
>>>> be).
>>> 
>>> Aah, that sounds entirely dodgy and possibly quite broken. We freed that
>>> init code, so BPF or your trampolines (or a tiny module) could actually
>>> fit in there and insert their own kallsyms, and then we have overlapping
>>> symbols, which would be pretty bad.
>>> 
>>> I thought the ftrace kallsym stuff was for the trampolines, which would
>>> be fairly similar to what BPF is doing. And why I'm trying to get a
>>> generic dynamic kallsym thing sorted. There's bound the be other
>>> code-gen things at some point.
>> 
>> Hi Peter, 
>> 
>> I guess you are looking for something for all ksym add/delete events, like;
>> 
>>      /*
>>       * PERF_RECORD_KSYMBOL
>>       *
>>       * struct {
>>       *      struct perf_event_header header;
>>       *      u64                             addr;
>>       *      u32                             len;
>>       *      u16                             ksym_type;
>>       *      u16                             flags;
>>       *      char                            name[];
>>       *      struct sample_id                sample_id;
>>       * };
>>       */
> 
> Can't this reuse PERF_RECORD_MMAP2 with some bit in the header to mean
> that the name is the symbol name, not a path to some ELF/whatever? The
> ksym type could be encoded in the prot field, PROT_EXEC for functions,
> PROT_READ for read only data, PROT_WRITE for rw data.

Thanks Arnaldo!

I think this works. PERF_RECORD_MMAP2 has many bits in it. We can encode
a lot of details. We can even have bit to differentiate map/unmap. 

> 
> If we do it that way older tools will show the DSO name and an
> unresolved symbol, and even an indication if its a function or data,
> which is better than not showing anything when processing a new
> PERF_RECORD_KSYMBOL.

For compatibility, we can use attr.bpf_event bit (or attr.mmap2_plus)
to turn on/off new variations of PERF_RECORD_MMAP2. Unless user runs
perf-record and perf-report with different versions of perf tools, we 
should not see weird events. 

> 
> New tools, seeing the perf_event_attr.header bit will know that this is
> a "map" with just one symbol and will show that for both DSO name and
> symbol.
> 

Hi Peter, 

Could you please share your comments/suggestions on Arnaldo's proposal?

Thanks,
Song