linux-kernel - Re: [PATCH] perf report: Do not process non-JIT BPF ksymbol events

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Message-ID: <bb2b30a1-8ce3-4565-b17a-27148234c10b@intel.com>
Date: Thu, 6 Mar 2025 08:48:20 +0200
From: Adrian Hunter <adrian.hunter@...el.com>
To: Namhyung Kim <namhyung@...nel.org>
CC: Arnaldo Carvalho de Melo <acme@...nel.org>, Ian Rogers
	<irogers@...gle.com>, Kan Liang <kan.liang@...ux.intel.com>, Jiri Olsa
	<jolsa@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar
	<mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	<linux-perf-users@...r.kernel.org>, <bpf@...r.kernel.org>, Kevin Nomura
	<nomurak@...gle.com>, Song Liu <song@...nel.org>
Subject: Re: [PATCH] perf report: Do not process non-JIT BPF ksymbol events

On 6/03/25 08:45, Namhyung Kim wrote:
> Hello,
> 
> On Thu, Mar 06, 2025 at 08:25:01AM +0200, Adrian Hunter wrote:
>> On 6/03/25 01:28, Namhyung Kim wrote:
>>> The length of PERF_RECORD_KSYMBOL for BPF is a size of JITed code so
>>> it'd be 0 when it's not JITed.  The ksymbol is needed to symbolize the
>>> code when it gets samples in the region but non-JITed code cannot get
>>> samples.  Thus it'd be ok to ignore them.
>>>
>>> Actually it caused a performance issue in the perf tools on old ARM
>>> kernels where it can refuse to JIT some BPF codes.  It ended up
>>> splitting the existing kernel map (kallsyms).  And later lookup for a
>>> kernel symbol would create a new kernel map from kallsyms and then
>>> split it again and again. :(
>>>
>>> Probably there's a bug in the kernel map/symbol handling in perf tools.
>>> But I think we need to fix this anyway.
>>>
>>> Reported-by: Kevin Nomura <nomurak@...gle.com>
>>> Cc: Song Liu <song@...nel.org>
>>> Signed-off-by: Namhyung Kim <namhyung@...nel.org>
>>> ---
>>>  tools/perf/util/machine.c | 4 ++++
>>>  1 file changed, 4 insertions(+)
>>>
>>> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
>>> index 3f1faf94198dbe56..c7d27384f0736408 100644
>>> --- a/tools/perf/util/machine.c
>>> +++ b/tools/perf/util/machine.c
>>> @@ -779,6 +779,10 @@ int machine__process_ksymbol(struct machine *machine __maybe_unused,
>>>  	if (dump_trace)
>>>  		perf_event__fprintf_ksymbol(event, stdout);
>>>  
>>> +	/* no need to process non-JIT BPF as it cannot get samples */
>>> +	if (event->ksymbol.len == 0)
>>> +		return 0;
>>
>> Are all ksymbol events BPF?  Maybe it is OK
>> for PERF_RECORD_KSYMBOL_TYPE_OOL also.  Perhaps adjust the
>> comment in that case.
> 
> Probably, but I didn't see OOL with zero length yet.  Is it possible?

Probably not