Re: [PATCH 3/4] perf map: Set kmap->kmaps backpointer for main kernel map chunks

[Ravi Bangoria <ravi.bangoria@linux.ibm.com>]

Hi Arnaldo,

On 12/23/19 7:02 PM, Arnaldo Carvalho de Melo wrote:
> From: Arnaldo Carvalho de Melo <acme@redhat.com>
> 
> When a map is create to represent the main kernel area (vmlinux) with
> map__new2() we allocate an extra area to store a pointer to the 'struct
> maps' for the kernel maps, so that we can access that struct when
> loading ELF files or kallsyms, as we will need to split it in multiple
> maps, one per kernel module or ELF section (such as ".init.text").
> 
> So when map->dso->kernel is non-zero, it is expected that
> map__kmap(map)->kmaps to be set to the tree of kernel maps (modules,
> chunks of the main kernel, bpf progs put in place via
> PERF_RECORD_KSYMBOL, the main kernel).
> 
> This was not the case when we were splitting the main kernel into chunks
> for its ELF sections, which ended up making 'perf report --children'
> processing a perf.data file with callchains to trip on
> __map__is_kernel(), when we press ENTER to see the popup menu for main
> histogram entries that starts at a symbol in the ".init.text" ELF
> section, e.g.:
> 
> -    8.83%     0.00%  swapper     [kernel.vmlinux].init.text  [k] start_kernel
>       start_kernel
>       cpu_startup_entry
>       do_idle
>       cpuidle_enter
>       cpuidle_enter_state
>       intel_idle
> 
> Fix it.

perf top from perf/core has started crashing at __map__is_kernel():

   (gdb) bt
   #0  __map__is_kernel (map=<optimized out>) at util/map.c:935
   #1  0x000000000045551d in perf_event__process_sample (machine=0xbab8f8,
       sample=0x7fffe5ffa6d0, evsel=0xba7570, event=0xbcac50, tool=0x7fffffff84e0)
       at builtin-top.c:833
   #2  deliver_event (qe=<optimized out>, qevent=<optimized out>) at builtin-top.c:1192
   #3  0x000000000050b9fb in do_flush (show_progress=false, oe=0x7fffffff87e0)
       at util/ordered-events.c:244
   #4  __ordered_events__flush (oe=oe@entry=0x7fffffff87e0, how=how@entry=OE_FLUSH__TOP,
       timestamp=timestamp@entry=0) at util/ordered-events.c:323
   #5  0x000000000050c1b5 in __ordered_events__flush (timestamp=<optimized out>,
       how=<optimized out>, oe=<optimized out>) at util/ordered-events.c:339
   #6  ordered_events__flush (how=OE_FLUSH__TOP, oe=0x7fffffff87e0) at util/ordered-events.c:341
   #7  ordered_events__flush (oe=oe@entry=0x7fffffff87e0, how=how@entry=OE_FLUSH__TOP)
       at util/ordered-events.c:339
   #8  0x0000000000454e21 in process_thread (arg=0x7fffffff84e0) at builtin-top.c:1104
   #9  0x00007ffff7f2c4e2 in start_thread () from /lib64/libpthread.so.0
   #10 0x00007ffff76086d3 in clone () from /lib64/libc.so.6

I haven't debugged it much but seems like the actual patch that's causing the
crash is de90d513b246 ("perf map: Use map->dso->kernel + map__kmaps() in
map__kmaps()").

Did you face this / aware of it?

Ravi