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

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Message-ID: <b33a432f-b6a5-e9e8-a744-f29c21c69fd8@redhat.com>
Date:   Tue, 13 Aug 2019 09:55:16 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Cc:     "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 13/08/19 02:55, Wanpeng Li wrote:
>> I think KVM_HINTS_REALTIME is being abused somewhat.
>> It has no clear meaning and used in different locations
>> for different purposes.
> 
> 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.

Guest side polling definitely matches the purpose of KVM_HINTS_REALTIME.
 While host-side polling is conditional on single_task_running, this is
obviously not true of guest-side polling.

The alternative would be to enable it only if KVM_FEATURE_POLL_CONTROL
is available, but I prefer Wanpeng's patch.

Paolo

>> 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
>