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

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Message-ID: <20190829143904.GA13983@amt.cnet>
Date:   Thu, 29 Aug 2019 11:39:06 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        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 09:53:04AM -0300, Marcelo Tosatti wrote:
> On Thu, Aug 29, 2019 at 08:16:41PM +0800, Wanpeng Li wrote:
> > > 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.
> > 
> > non overcommit group, if they don't care latency/performance, they
> > don't need to enable haltpoll, "realtime hints" etc. Otherwise, they
> > should better tune.
> 
> As mentioned before, "being overcommitted" is a property which is transitional.
> 
> A static true/false scheme reflects this poorly.
> 
> Therefore the OS should detect it and act accordingly.

Hi Wanpeng Li,

One suggestion for a dynamic "is overcommited" scheme:

If the amount of stolen time, in the past record_steal_time window, 
is more than 20% of the time in that window, then mark system
as overcommitted. Otherwise, clear it.

Make that 20% configurable by as kvm module parameter.

Use that info to enable/disable overcommit features.

That should work, right?