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)

Peter Zijlstra
2010-10-25T10:13:12Z

On Fri, 2010-10-22 at 16:03 -0700, Venkatesh Pallipadi wrote:
> I started making small changes to the code, but none of the change helped much.
> I think the problem with the current code is that, even though idle CPUs
> update load, the fold only happens when one of the CPU is busy
> and we end up taking its load into global load.
>
> So, I tried to simplify things and doing the updates directly from idle loop.
> This is only a test patch, and eventually we need to hook it off somewhere
> else, instead of idle loop and also this is expected work only as x86_64
> right now.
>
> Peter: Do you think something like this will work? loadavg went
> quite on two of my test systems after this change (4 cpu and 24 cpu).

Not really, CPUs can stay idle for _very_ long times (!x86 cpus that
don't have crappy timers like HPET which roll around every 2-4 seconds).

But all CPUs staying idle for a long time is exactly the scenario you
fix before using the decay_load_misses() stuff, except that is for the
load-balancer per-cpu load numbers not the global cpu load avg. Won't a
similar approach work here?