linux-kernel - Re: [PATCH 2/4] perf report: Load PE files from debug cache only

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Date:   Fri, 12 Feb 2021 09:28:00 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Nicholas Fraser <nfraser@...eweavers.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        "Frank Ch. Eigler" <fche@...hat.com>,
        Song Liu <songliubraving@...com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kim Phillips <kim.phillips@....com>,
        Tommi Rantala <tommi.t.rantala@...ia.com>,
        Remi Bernon <rbernon@...eweavers.com>,
        linux-kernel@...r.kernel.org,
        Ulrich Czekalla <uczekalla@...eweavers.com>,
        Huw Davies <huw@...eweavers.com>
Subject: Re: [PATCH 2/4] perf report: Load PE files from debug cache only

Em Wed, Feb 10, 2021 at 02:17:38PM -0500, Nicholas Fraser escreveu:
> dso__load_bfd_symbols() attempts to load a DSO at its original path,
> then closes it and loads the file in the debug cache. This is incorrect.
> It should ignore the original file and work with only the debug cache.
> The original file may have changed or may not even exist, for example if
> the debug cache has been transferred to another machine via "perf
> archive".
> 
> This fix makes it only load the file in the debug cache.

Well this improves your current use case and only affects PE files, so I
am applying, but consider a slightly different workflow:

 1. perf record ./foo.exe
 2. perf report     # works, finds the file in the ~/.debug cache, as stored
                    # by 'perf record'
 3. rm -rf ~/.debug # I need more space
 4. perf report     # Fails, as it looks only in the ~/.debug cache, that
                    # was nuked


So at 4 it should look at the original pathname, and hope for the best.

All this is moot if we have something like a build-id in PE files,
where we can look in any order since we'll verify the unique ID to see
if it is the one we need, right?

- Arnaldo
 
> Signed-off-by: Nicholas Fraser <nfraser@...eweavers.com>
> ---
>  tools/perf/util/symbol.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
> index 64a039cbba1b..aa9ae875b995 100644
> --- a/tools/perf/util/symbol.c
> +++ b/tools/perf/util/symbol.c
> @@ -1569,7 +1569,7 @@ int dso__load_bfd_symbols(struct dso *dso, const char *debugfile)
>  	u_int i;
>  	u64 start, len;
>  
> -	abfd = bfd_openr(dso->long_name, NULL);
> +	abfd = bfd_openr(debugfile, NULL);
>  	if (!abfd)
>  		return -1;
>  
> @@ -1586,12 +1586,6 @@ int dso__load_bfd_symbols(struct dso *dso, const char *debugfile)
>  	if (section)
>  		dso->text_offset = section->vma - section->filepos;
>  
> -	bfd_close(abfd);
> -
> -	abfd = bfd_openr(debugfile, NULL);
> -	if (!abfd)
> -		return -1;
> -
>  	if (!bfd_check_format(abfd, bfd_object)) {
>  		pr_debug2("%s: cannot read %s bfd file.\n", __func__,
>  			  debugfile);
> -- 
> 2.30.0
> 

-- 

- Arnaldo