[Xen-devel] Re: [PATCH] cpuidle: suspend/resume scheduler tick timer during cpu idle entry/exit

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 20/03/2009 12:51, "Yu, Ke" <ke.yu@xxxxxxxxx> wrote:

> cpuidle can collaborate with scheduler to reduce unnecessary timer interrupt.
> For example, credit scheduler accounting timer doesn't need to be active at
> idle time, so it can be stopped at cpuidle entry and resumed at cpuidle exit.
> This patch implements this function by adding two ops in scheduler:
> tick_suspend/tick_resume, and implement them for credit scheduler
> 
> With this patch, under idle scenario, timer interrupt frequency decreased from
> ~100HZ to ~10HZ, and average C state residency increase from ~10ms to larger
> than 100ms. Also in a two-socket machine, about 4% idle power saving is
> observed. 
> 
> However, one issue is observed with this patch, i.e. there is soft-lockup in
> dom0 occasionally. This issue is still under debugging. Currently we already
> find a >1s VCPUOP_set_singleshot_timer timeout, which imply this may be a dom0
> issue. we are working hard to figure the root cause.

I don't really want to take the patch while it is soft locking up. I would
expect linux-2.6.18-xen.hg:22 to avoid lockup warnings due to too long
singleshot timeouts (I assume you are testing with the 2.6.18 tree?).

Personally I would rather have cpuidle be enabled by default (or even always
with no disable option) and get existing Cx benefits for everyone, rather
than have a slightly broken cpuidle option.

Is there a reason not to turn on cpuidle by default now? Or even enable and
then remove the boot option?

 -- Keir



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