On Wed, Nov 28, 2018 at 08:35:34AM -0800, Eric Saint-Etienne wrote: > When the kernel is compiled with -ffunction-sections and perf uses the > kernel debuginfo, perf fails the very first symbol lookup and ends up with > an hex offset inside [kernel.vmlinux]. It's due to how perf loads the maps. > > Indeed only .text gets loaded by map_groups__find() into al->map. > Consequently al->map address range encompass the whole kernel image. > But then map__load() loads many function maps by splitting al->map, > which reduces al->map range drastically. Very likely the target address is > then in one of those newly created function maps, so we need to lookup the > map again to find that new map. > > I'm not sure if this issue is only specific to the kernel but at least it > occurs withe the kernel dso, and when we're not using the kernel debuginfo, > perf will fallback to using kallsyms and then the first lookup will work. > > The split of .text section happens in dso_process_kernel_symbol() where we > call map_groups__find_by_name() to find an existing map, but with > -ffunction-sections and a symbol belonging to a new (function) map, such > map doesn't exist yet so we end up creating one and adjusting existing maps > accordingly because adjust_kernel_syms is set there. > > This patch makes sure that the event address we're looking-up is indeed > within the map we've found, otherwise we lookup another map again. > Only one extra lookup at most is required for the proper map to be found, > if it exists. > > Signed-off-by: Eric Saint-Etienne <[email protected]> > Reviewed-by: Darren Kenny <[email protected]>
Reviewed-by: Jiri Olsa <[email protected]> thanks, jirka
