Messages in this thread | | | Date | Wed, 25 Jan 2023 09:37:59 +0200 | Subject | Re: [PATCH] perf/util: Symbol lookup can fail if multiple segmets match stext | From | Adrian Hunter <> |
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On 25/01/23 09:29, Adrian Hunter wrote:
Also subject line has spelling mistake, and should identify kcore as the issue e.g.
perf symbol: Symbol lookup with kcore can fail if multiple segments match stext
> On 25/01/23 00:35, Krister Johansen wrote: >> This problem was encountered on an arm64 system with a lot of memory. >> Without kernel debug symbols installed, and with both kcore and kallsyms >> available, perf managed to get confused and returned "unknown" for all >> of the kernel symbols that it tried to look up. >> >> On this system, stext fell within the vmalloc segment. The kcore symbol >> matching code tries to find the first segment that contains stext and >> uses that to replace the segment generated from just the kallsyms >> information. In this case, however, there were two: a very large >> vmalloc segment, and the text segment. This caused perf to get confused >> because multiple overlapping segments were inserted into the RB tree >> that holds the discovered segments. However, that alone wasn't >> sufficient to cause the problem. Even when we could find the segment, >> the offsets were adjusted in such a way that the newly generated symbols >> didn't line up with the instruction addresses in the trace. The most >> obvious solution would be to consult which segment type is text from >> kcore, but this information is not exposed to users. >> >> Instead, select the smallest matching segment that contains stext >> instead of the first matching segment. This allows us to match the text >> segment instead of vmalloc, if one is contained within the other. >> >> Signed-off-by: Krister Johansen <kjlx@templeofstupid.com> >> --- >> tools/perf/util/symbol.c | 10 ++++++++-- >> 1 file changed, 8 insertions(+), 2 deletions(-) >> >> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c >> index a3a165ae933a..14ac4189eaff 100644 >> --- a/tools/perf/util/symbol.c >> +++ b/tools/perf/util/symbol.c >> @@ -1368,10 +1368,16 @@ static int dso__load_kcore(struct dso *dso, struct map *map, >> >> /* Find the kernel map using the '_stext' symbol */ >> if (!kallsyms__get_function_start(kallsyms_filename, "_stext", &stext)) { >> + u64 replacement_size = 0; > > We'd usually put a blank line here > >> list_for_each_entry(new_map, &md.maps, node) { >> - if (stext >= new_map->start && stext < new_map->end) { >> + u64 new_size = new_map->end - new_map->start; >> + >> + if (!(stext >= new_map->start && stext < new_map->end)) >> + continue; >> + > > Really needs a comment, and please be specific e.g. > > ARM64 vmalloc segment overlaps the kernel text segment, so > choosing the smaller segment will get the kernel text. > > > >> + if (!replacement_map || new_size < replacement_size) { >> replacement_map = new_map; >> - break; >> + replacement_size = new_size; >> } >> } >> } >
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