perf confused by modules being loaded in between addresses in /proc/kallsyms

From: Michael Hudson-Doyle
Date: Tue Oct 29 2013 - 20:53:50 EST


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.

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