DLPX-90487 sdb stacks appears to loop resulting in many repeated stack frames

[mmaybee]

Problem

Often, when using the stacks command, the stack listing will be very long
(1024 lines) with most of the stack being 0x0+0x0 or some other "cruft":

0xffff9debf71ecc80 INTERRUPTIBLE       493
                  __schedule+0x2c8
                  __schedule+0x2c8
                  schedule+0x69
                  schedule_timeout+0x18f
                  svc_recv+0x45e
                  svc_recv+0x45e
                  nfsd+0xdb
                  kthread+0x127
                  ret_from_fork+0x1f
                  0x0+0x0
                  0x0+0x0
                  0x0+0x0
                  0x0+0x0
                  0x0+0x0
                  0x0+0x0
                  0x0+0x0
                  ...

Evaluation

The issue does not appear to be in the stack trace output code, but rather
in the drgn code that generates the array of stack task pointers. Sometimes
this array appears to be non-terminated and instead has the max length
(of 1024) with “cruft” (usually zeros) padding out the end after the stack.

Solution

The stack printing algorithm has been modified to ignore task frames that
have a program counter value of 0x0. The algorithm will also aggregate
adjacent frames where the program counter and offset are the same:

0xffff9debf71ecc80 INTERRUPTIBLE       493
                  __schedule+0x2c8 (2)
                  schedule+0x69
                  schedule_timeout+0x18f
                  svc_recv+0x45e (2)
                  nfsd+0xdb
                  kthread+0x127
                  ret_from_fork+0x1f

Testing Done

Verified that the stack prints as expected on a system where the old
algorithm printed long stacks.

[tonynguien]

Looks reasonable to me.