'autogroup' sched code KILLING responsiveness

[Michael Witten]

Bisecting shows that this commit:

  5091faa449ee0b7d73bc296a93bca9540fc51d0a
  sched: Add 'autogroup' scheduling feature: automated per session task groups
  Date:   Tue Nov 30 14:18:03 2010 +0100

is the reason that my computer has become unusable.

With that code in place, a resource-intensive activity (such as
compiling the Linux kernel) causes my computer to become
unresponsive for many seconds at a time; the entire screen
does not refresh, typed keys are dropped or are handled very
late, etc (even in Linux's plain virtual consoles).

I'm using a uniprocessor (UP) machine, and I've noticed that such
codepaths often get clobbered by changes that only make it to SMP
configurations (I guess kernel hackers have better equipment than
I do); maybe that has something to do with it.

I'm a laymen, so all I can do at this time is report my experience;
if this has already been discussed, I would appeciate a link to
the right thread.

Sincerely,
Michael Witten
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