[Bug 513848] Re: [karmic] CPU load not being reported accurately

[Chase Douglas]

Stable Release Update Justification:

Impact of bug: On low load systems with specific work load
characteristics the load average may not be representative of the real
load. Given a specific test case it is possible to load a system with
NR_CPUS number of tasks and yet have a load avg of 0.00, though this is
unlikely to occur in real work loads.

How addressed: The attached patch reworks the load accounting mechanism
in the kernel scheduler. It ensures that the accounting is strictly
dependent on the time (i.e. snapshot taken every 5 seconds) and the
number of runnable and uninterruptible tasks at that given time.
Previously, the accounting also depended on whether a cpu goes idle
shortly after the 5 second snapshot.

Reproduction: See attached reproduction test case. Run it once on a non-
loaded system (boot to rescue mode works well). Top will report the cpu
usage at 90%, but uptime will report a load avg near 0.00 instead of at
least 0.90 as expected.

Regression potential: The patch has been received well from senior
Ubuntu kernel team members and some of the upstream kernel maintainers
on lkml. For this reason it is assumed to be a good fix for this issue.
The only code path touched by this patch involves the load avg
accounting, so potential regressions could include incorrect load avg
and/or some unforeseen general bug like a null dereference. However, the
likelihood of either is minimal due to proper and thorough patch review.

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[karmic] CPU load not being reported accurately
https://bugs.launchpad.net/bugs/513848
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