linux-kernel - Re: [PATCH v3 3/3] perf maps: Remove the replacement of kernel map

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Date: Tue, 28 May 2024 16:33:12 +0100
From: James Clark <james.clark@....com>
To: Leo Yan <leo.yan@....com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
 Ian Rogers <irogers@...gle.com>, Namhyung Kim <namhyung@...nel.org>,
 Adrian Hunter <adrian.hunter@...el.com>,
 Athira Rajeev <atrajeev@...ux.vnet.ibm.com>,
 Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, linux-perf-users@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] perf maps: Remove the replacement of kernel map



On 20/05/2024 10:06, Leo Yan wrote:
> The kernel text map has been removed from the kernel maps by
> maps__remove_maps(), and the kcore maps are organized in order, allowing
> us to achieve neat kernel maps.
> 
> As a result, it is not necessary to replace the old kernel text map.
> Instead, the commit finds the latest text map, assigns it to
> 'machine->vmlinux_map', and releases the old map.
> 
> One concern is if a platform fails to find a kernel text map after
> updating maps list with kcore, in this case, it should not happen and
> reports the failure.
> 

Hi Leo,

For some reason this commit causes the symbol to not be resolved for
one kernel sample in a recording I have. It's a bit weird because the
same address is resolved in other samples.

  $ perf script

Before this commit:

  perf-exec ffff80008030cfd4 perf_event_exec+0x22c ([kernel.kallsyms])
  perf-exec ffff80008030cfd4 perf_event_exec+0x22c ([kernel.kallsyms])
  perf-exec ffff80008030cfd4 perf_event_exec+0x22c ([kernel.kallsyms])
  perf-exec ffff80008129a0f8 _raw_spin_unlock_irqrestore+0x48
    ([kernel.kallsyms])
  ls ffff80008012f5ec lock_acquire+0x74 ([kernel.kallsyms])

After:

  perf-exec ffff80008030cfd4 [unknown] ([kernel.kallsyms])
  perf-exec ffff80008030cfd4 perf_event_exec+0x22c ([kernel.kallsyms])
  perf-exec ffff80008030cfd4 perf_event_exec+0x22c ([kernel.kallsyms])
  perf-exec ffff80008129a0f8 _raw_spin_unlock_irqrestore+0x48
    ([kernel.kallsyms])
  ls ffff80008012f5ec lock_acquire+0x74 ([kernel.kallsyms])

James