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Sometimes it matches a variable in the inner scope but it fails because
the actual access can be on a different type.  Let's try variables in
every scope and choose the best one using is_better_type().

I have an example with update_blocked_averages(), at first it found a
variable (__mptr) but it's a void pointer.  So it moved on to the upper
scope and found another variable (cfs_rq).

  $ perf --debug type-profile annotate --data-type --stdio
  ...
  -----------------------------------------------------------
  find data type for 0x140(reg14) at update_blocked_averages+0x2db
  CU for kernel/sched/fair.c (die:0x12dd892)
  frame base: cfa=1 fbreg=7
  found "__mptr" (die: 0x13022f1) in scope=4/4 (die: 0x13022e8) failed: no/void pointer
   variable location: base=reg14, offset=0x140
   type='void*' size=0x8 (die:0x12dd8f9)
  found "cfs_rq" (die: 0x1301721) in scope=3/4 (die: 0x130171c) type_offset=0x140
   variable location: reg14
   type='struct cfs_rq' size=0x1c0 (die:0x12e37e5)
  final type: type='struct cfs_rq' size=0x1c0 (die:0x12e37e5)

IIUC the scope is like below:
  1: update_blocked_averages
  2:   __update_blocked_fair
  3:     for_each_leaf_cfs_rq_safe
  4:       list_entry -> (container_of)

The container_of is implemented like:

  #define container_of(ptr, type, member) ({				\
  	void *__mptr = (void *)(ptr);					\
  	static_assert(__same_type(*(ptr), ((type *)0)->member) ||	\
  		      __same_type(*(ptr), void),			\
  		      "pointer type mismatch in container_of()");	\
  	((type *)(__mptr - offsetof(type, member))); })

That's why we see the __mptr variable first but it failed since it has
no type information.

Then for_each_leaf_cfs_rq_safe() is defined as

  #define for_each_leaf_cfs_rq_safe(rq, cfs_rq, pos)			\
  	list_for_each_entry_safe(cfs_rq, pos, &rq->leaf_cfs_rq_list,	\
  				 leaf_cfs_rq_list)

Note that the access was 0x140(r14).  And the cfs_rq has
leaf_cfs_rq_list at the 0x140.  So it converts the list_head pointer to
a pointer to struct cfs_rq here.

  $ pahole --hex -C cfs_rq vmlinux | grep 140
  struct cfs_rq 	struct list_head           leaf_cfs_rq_list;     /* 0x140  0x10 */
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240816235840.2754937-9-namhyung@kernel.org

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>