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

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Message-ID: <d962792a-c852-494b-b35c-e8f83cac7218@intel.com>
Date: Thu, 6 Mar 2025 08:25:01 +0200
From: Adrian Hunter <adrian.hunter@...el.com>
To: Namhyung Kim <namhyung@...nel.org>, Arnaldo Carvalho de Melo
	<acme@...nel.org>, Ian Rogers <irogers@...gle.com>, Kan Liang
	<kan.liang@...ux.intel.com>
CC: 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 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.

> +
>  	if (event->ksymbol.flags & PERF_RECORD_KSYMBOL_FLAGS_UNREGISTER)
>  		return machine__process_ksymbol_unregister(machine, event,
>  							   sample);