linux-kernel - Re: [PATCH] cpuidle-haltpoll: Enable kvm guest polling when dedicated physical CPUs are available

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Date:   Wed, 28 Aug 2019 11:48:58 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH] cpuidle-haltpoll: Enable kvm guest polling when
 dedicated physical CPUs are available

On Tue, Aug 27, 2019 at 08:43:13AM +0800, Wanpeng Li wrote:
> > > kvm adaptive halt-polling will compete with
> > > vhost-kthreads, however, poll in guest unaware other runnable tasks in
> > > the host which will defeat vhost-kthreads.
> >
> > It depends on how much work vhost-kthreads needs to do, how successful
> > halt-poll in the guest is, and what improvement halt-polling brings.
> > The amount of polling will be reduced to zero if polling
> > is not successful.
> 
> We observe vhost-kthreads compete with vCPUs adaptive halt-polling in
> kvm, it hurt performance in over-subscribe product environment,
> polling in guest can make it worse.
> 
> Regards,
> Wanpeng Li

Wanpeng,

Polling should not be performed if there is other work to do. For
example, halt-polling could check a host/guest shared memory 
region indicating whether there are other runnable tasks in the host.

Disabling polling means you will not achieve the improvement 
even in the transitional periods where the system is not
overcommitted (which should be frequent given that idling 
is common).

Again, about your patch: it brings no benefit to anyone. 

Guest halt polling should be already disabled by default
(the driver has to be loaded for guest polling to take place).