Re: High CPU load when machine is idle (related to PROBLEM: Unusually high load average when idle in 2.6.35, 2.6.35.1 and later)

Damien Wyart
2010-10-14T14:58:38Z

Hello,

> With 2.6.36-rc6, I'm seeing a load of around 0.60 when the machine is
> completely idle. This is similar to what someone reported for the latest
> 2.6.35.x stables. This is on a core i7 machine, but I've no time to
> bisect or test earlier versions right now, but I guess this is easy to
> reproduce on the same plateform.

After further investigation and cross-checking with the thread "PROBLEM:
Unusually high load average when idle in 2.6.35, 2.6.35.1 and later",
I came to the following results:

- the commit 74f5187ac873042f502227701ed1727e7c5fbfa9 isolated by Tim
seems to be the culprit;
- reverting it solves the problem with 2.6.36-rc7 in NOHZ mode: the load
when idle goes down to 0.00 (which it never does with the patch
applied)
- using nohz=no with the commit reverted still gives correct behaviour
(tested just in case)

- vanilla 2.6.36-rc7 with this commit applied has the problem (load is
around 0.60 when machine idle, sometimes less after several hours
of uptime, but never 0.00), and rebooting with nohz=no makes the
problem disappear: load goes down to 0.00 quickly after boot process
has finished.

I hope this answers the questions raised in the Tim's thread.

Could someone with knowledge of the commit take a look at the problem?
It would be a bit annoying to have this problem in 2.6.36, since Tim's
initial report dates back to 2 weeks ago...

I can help for further testing if needed.

Thanks in advance,
--
Damien