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

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Message-ID: <20190803202058.GA9316@amt.cnet>
Date:   Sat, 3 Aug 2019 17:21:01 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        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 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

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

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

Wanpeng Li, currently the driver does not autoload. So polling in 
the guest has to be enabled manually. Isnt that sufficient?