Messages in this thread | | | From | Michael Hudson-Doyle <> | Subject | perf confused by modules being loaded in between addresses in /proc/kallsyms | Date | Tue, 29 Oct 2013 17:53:39 -0700 |
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Hi,
On arm, probably since "ARM: use linker magic for vectors and vector stubs" (although I haven't checked this), perf report tends to allocate all kernel time to the module loaded at the highest address.
This turns out to be because the "perf_event__synthesize_modules" generates a PERF_RECORD_MMAP for the last module thats runs from its start to the end of the address space.
This in turn turns out be because the first symbol in /proc/kallsyms is now at 0:
# head -n 2 /proc/kallsyms 00000000 t __vectors_start c00081c0 T asm_do_IRQ
/This/ means that the kallsyms entry in machine->kmaps[MAP__FUNCTION] is now the first entry rather than the last in the map, so the last module is the last in the map and so its "end" stays as ~0ULL.
I'm told that there is no particularly good reason for __vectors_start to be in kallsyms at 0, but in any case this seems like a bug in the user space tool. I notice that there is code to split the kallsyms symbols around symbols from modules (dso__split_kallsyms) but this doesn't seem to be called for record, only report. Is the fix as simple as making sure this happens for record as well? I don't feel qualified to answer that.
Cheers, mwh
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