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

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Message-ID: <CANRm+CwtHBOVWFcn+6Z3Ds7dEcNL2JP+b6hLRS=oeUW98A24MQ@mail.gmail.com>
Date:   Tue, 13 Aug 2019 08:55:29 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Marcelo Tosatti <mtosatti@...hat.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>
Subject: Re: [PATCH] cpuidle-haltpoll: Enable kvm guest polling when dedicated
 physical CPUs are available

On Sun, 4 Aug 2019 at 04:21, Marcelo Tosatti <mtosatti@...hat.com> wrote:
>
> On Thu, Aug 01, 2019 at 06:54:49PM +0200, Paolo Bonzini wrote:
> > On 01/08/19 18:51, Rafael J. Wysocki wrote:
> > > On 8/1/2019 9:06 AM, Wanpeng Li wrote:
> > >> From: Wanpeng Li <wanpengli@...cent.com>
> > >>
> > >> The downside of guest side polling is that polling is performed even
> > >> with other runnable tasks in the host. However, even if poll in kvm
> > >> can aware whether or not other runnable tasks in the same pCPU, it
> > >> can still incur extra overhead in over-subscribe scenario. Now we can
> > >> just enable guest polling when dedicated pCPUs are available.
> > >>
> > >> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > >> Cc: Paolo Bonzini <pbonzini@...hat.com>
> > >> Cc: Radim Krčmář <rkrcmar@...hat.com>
> > >> Cc: Marcelo Tosatti <mtosatti@...hat.com>
> > >> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > >
> > > Paolo, Marcelo, any comments?
> >
> > Yes, it's a good idea.
> >
> > Acked-by: Paolo Bonzini <pbonzini@...hat.com>
> >
> > Paolo
>

Hi Marcelo,

Sorry for the late response.

> I think KVM_HINTS_REALTIME is being abused somewhat.
> It has no clear meaning and used in different locations
> for different purposes.

================== ============ =================================
KVM_HINTS_REALTIME 0                      guest checks this feature bit to

determine that vCPUs are never

preempted for an unlimited time

allowing optimizations
================== ============ =================================

Now it disables pv queued spinlock, pv tlb shootdown, pv sched yield
which are not expected present in vCPUs are never preempted for an
unlimited time scenario.

>
> For example, i think that using pv queued spinlocks and
> haltpoll is a desired scenario, which the patch below disallows.

So even if dedicated pCPU is available, pv queued spinlocks should
still be chose if something like vhost-kthreads are used instead of
DPDK/vhost-user. 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.

Regards,
Wanpeng Li