scripts/gdb: fix detection of current CPU in KGDB

Directly read the current CPU number from the kgdb_active variable.

Before, the active CPU was obtained through the current task, which
required searching the task list for the pid of GDB's selected thread.
Obtaining the pid was buggy: GDB may use selected_thread().ptid[1] (LWPID)
instead of .ptid[2] (TID) to store the threads pid; see
https://sourceware.org/gdb/current/onlinedocs/gdb.html/Threads-In-Python.html
As a result, the detection could return the wrong CPU number, leading to
incorrect results for $lx_per_cpu and $lx_current.

As a side effect, the patch significantly speeds up $lx_per_cpu and
$lx_current in KGDB by avoiding the task-list iteration.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Florian Rommel <[e-mail]>
Cc: Andrew Jones <[e-mail]>
Cc: Deepak Gupta <[e-mail]>
Cc: Jan Kiszka <[e-mail]>
Cc: Kieran Bingham <[e-mail]>
Cc: Kuan-Ying Lee <[e-mail]>
Cc: Palmer Dabbelt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

scripts/gdb/linux/cpus.py

···
     if utils.get_gdbserver_type() == utils.GDBSERVER_QEMU:
         return gdb.selected_thread().num - 1
     elif utils.get_gdbserver_type() == utils.GDBSERVER_KGDB:
-        tid = gdb.selected_thread().ptid[2]
-        if tid > (0x100000000 - MAX_CPUS - 2):
-            return 0x100000000 - tid - 2
-        else:
-            return tasks.get_thread_info(tasks.get_task_by_pid(tid))['cpu']
+        return gdb.parse_and_eval("kgdb_active.counter")
     else:
         raise gdb.GdbError("Sorry, obtaining the current CPU is not yet "
                            "supported with this gdb server.")