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

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Message-ID: <20190829120422.GC4949@amt.cnet>
Date:   Thu, 29 Aug 2019 09:04:22 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        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 Thu, Aug 29, 2019 at 01:37:35AM +0200, Rafael J. Wysocki wrote:
> On Wed, Aug 28, 2019 at 4:39 PM Marcelo Tosatti <mtosatti@...hat.com> wrote:
> >
> > On Wed, Aug 28, 2019 at 10:45:44AM +0200, Rafael J. Wysocki wrote:
> > > On Wed, Aug 28, 2019 at 10:34 AM Wanpeng Li <kernellwp@...il.com> wrote:
> > > >
> > > > On Tue, 27 Aug 2019 at 08:43, Wanpeng Li <kernellwp@...il.com> wrote:
> > > > >
> > > > > Cc Michael S. Tsirkin,
> > > > > On Tue, 27 Aug 2019 at 04:42, Marcelo Tosatti <mtosatti@...hat.com> wrote:
> > > > > >
> > > > > > On Tue, Aug 13, 2019 at 08:55:29AM +0800, Wanpeng Li wrote:
> > > > > > > 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>
> > > >
> > > > Hi Marcelo,
> > > >
> > > > If you don't have more concern, I guess Rafael can apply this patch
> > > > now since the merge window is not too far.
> > >
> > > I will likely queue it up later today and it will go to linux-next
> > > early next week.
> > >
> > > Thanks!
> >
> > NACK patch.
> 
> I got an ACK from Paolo on it, though.  Convince Paolo to withdraw his
> ACK if you want it to not be applied.
> 
> > Just don't load the haltpoll driver.
> 
> And why would that be better?

Split the group of all kvm users in two: overcommit group and non-overcommit
group.

Current situation regarding haltpoll driver is:

overcommit group: haltpoll driver is not loaded by default, they are
happy.

non overcommit group: boots without "realtime hints" flag, loads haltpoll driver, 
happy.

Situation with patch above:

overcommit group: haltpoll driver is not loaded by default, they are
happy.

non overcommit group: boots without "realtime hints" flag, haltpoll driver
cannot be loaded.