Re: [PATCH] x86: add cpuidle_kvm driver to allow guest side halt polling

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, May 22, 2019 at 05:44:34PM +0200, Paolo Bonzini wrote:
> On 22/05/19 17:04, Marcelo Tosatti wrote:
> > Consider sequence of wakeup events as follows:
> > 20us, 200us, 20us, 200us...
> 
> I agree it can happen, which is why the grow/shrink behavior can be
> disabled for halt_poll_ns.  Is there a real-world usecase with a
> sequence like this?

If you have a database with variable response times in the
20,200,20,200... range, then yes.

Its not a bizzarre/unlikely sequence.

You didnt answer my question at the end of the email.

> The main qualm I have with guest-side polling is that it encourages the
> guest admin to be "impolite".  But I guess it was possible even now to
> boot guests with idle=poll, which would be way more impolite...

Yep.

Thanks.

> Paolo
> 
> > If one enables guest halt polling in the first place,
> > then the energy/performance tradeoff is bend towards
> > performance, and such misses are harmful.
> > 
> > So going to add something along the lines of:
> > 
> > "If, after 50 consecutive times, block_time is much larger than
> > halt_poll_ns, then set cpu->halt_poll_ns to zero".
> > 
> > Restore user halt_poll_ns value once one smaller block_time
> > is observed.
> > 
> > This should cover the full idle case, and cause minimal
> > harm to performance.
> > 
> > Is that OK or is there any other characteristic of
> > adaptive halt poll you are looking for?