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

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Message-ID: <Z8lTzROPDcoPQXFZ@google.com>
Date: Wed, 5 Mar 2025 23:50:37 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Adrian Hunter <adrian.hunter@...el.com>
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 Thu, Mar 06, 2025 at 08:48:20AM +0200, Adrian Hunter wrote:
> 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

Then I think it's ok to leave the comment as is.

Thanks,
Namhyung