[Crash-utility] crash CPU bound waiting for user response

[D. Hugh Redelmeier]

On Thu, 5 Jul 2007, Dave Anderson wrote:

| From: Dave Anderson <anderson at redhat.com>

| D. Hugh Redelmeier wrote:
| > | From: Dave Anderson <anderson at redhat.com>
| > 
| > | D. Hugh Redelmeier wrote:
| > 
| > | > ==> Worse: while it is awaiting my RETURN, it is burning 100% of the
| > | > CPU!

| Again, what exactly do you do to reproduce it?  I just cannot get the 100%
| cpu-time waiting on the "less" sub-shell.

The simplest example, in a 24-line xterm:
    $ su
    # crash
    crash> help set

(I think that he su is necessary because crash is examining the live 
kernel.)

This behaviour comes up whenever crash is using more for more than a page.  
Except "crash --help" which seems to be different.

This machine is an Athlon X2 running an up-to-date x86_64 Fedora 7.
Mind you, only one core is enabled (because of a kernel bug that is
the motivation for using crash).

The CPU goes to 100%.  I presume that most of it is in the kernel,
handling the waitpid.

This seems really easy for me to reproduce.  What happens when you try
this?  In what environment?

| Anyway, I'm going to have to be able to reproduce it and test any
| changes thoroughly before potentially re-introducing the hangs I
| used to see.

Sure.  And my suggestion was not tested even by me.  It was only part
of an argument showing that the current code is wrong.

If you have a version of crash waiting for the pager to finish (as in my
example), put a gdb on it to find out just where in crash it is
waiting.  (I've told you where mine is waiting.)

I suspect that your crash would not be in a waitpid loop because that
would be a busy wait and you say you don't see a busy wait.