[Lucid] SRU: request pull EC2 Revert "sched: update load count only once per cpu in 10 tick update window"

[Chase Douglas]

On Fri, 2010-08-20 at 11:01 -0700, John Johansen wrote:
> The following changes since commit 65316c3b7e1fccaae5f2c0cf5f98cd3220a5be64:
>   Stefan Bader (1):
>         UBUNTU: Ubuntu-2.6.32-308.15
> 
> are available in the git repository at:
> 
>   git://kernel.ubuntu.com/jj/ubuntu-lucid ec2
> 
> John Johansen (1):
>       UBUNTU: SAUCE: Revert "sched: update load count only once per cpu in 10 tick update window"
> 
>  kernel/sched.c |   24 ++----------------------
>  1 files changed, 2 insertions(+), 22 deletions(-)
> 
> ------
> 
> UBUNTU: SAUCE: Revert "sched: update load count only once per cpu in 10 tick update window"
> 
> BugLink: http://bugs.launchpad.net/bugs/574910
> 
> SRU Justification:
> 
> Impact:
> Fixes loadavg reporting on EC2.
> 
> Fix:
> This reverts commit 0d843425672f4d2dc99b9004409aae503ef4d39f which fixes a bug in load
> accounting when a tickless (no idle HZ) kernel is used.  However the Xen patchset used
> on EC2 is not tickless but the accounting modifications are still being done, resulting
> in phantom load.
> 
> Testcase:
> Start any Ubuntu Lucid based instance on EC2, let it idle while logging the load average.
>      while true ; do cat /proc/loadavg >>load.log ; sleep 5 ; done
> Alternately simply run top or htop and monitor the load average.
> 
> Without the revert the reported load will vary from 0 up to about .5 for a clean image
> with no extra tasks launched.
> 
> With the revert the load stays steady around 0 with only occasional small bump when
> a background task is run.

On IRC, John said using the upstream version of this patch fixes the
issue, so I'm comfortable with swapping the patches as a resolution.

I'm more against removing this patch and not putting in the upstream
patch. However, I'll defer to John if he thinks the non-tickless Xen
kernel doesn't need this fix and it's a better solution for ec2.

Acked-by: Chase Douglas <chase.douglas at canonical.com>