linux-kernel - Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT

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Date:   Fri, 29 Sep 2017 10:18:25 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        mingo@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO
 hypercall\

On 28/09/2017 23:35, Marcelo Tosatti wrote:
> On Thu, Sep 28, 2017 at 09:22:02AM +0200, Paolo Bonzini wrote:
>> On 28/09/2017 02:44, Marcelo Tosatti wrote:
>>>> Again: if you have many interruptions, it's not a flaw in KVM or QEMU's
>>>> design, it's just that someone is doing something stupid.  It could be
>>>> the guest (e.g. unnecessary devices or daemons as in the example above),
>>>> QEMU (e.g. the RTC emulation used to trigger QEMU timers twice a second
>>>> just to increment the clock), or the management (e.g. polling "is the VM
>>>> running" 50 times per second).  But it can and must be fixed.
>>>
>>> No, i mean you can run anything in VCPU-0 (it is valid to do that).
>>> And that "anything" can generate 1 interrupt per second, 1000 or 10.000
>>> interrupts per second. Which are all valid things to be done.
>>>
>>> "I can't run a kernel compilation on VCPU-0 because that will impact
>>> latency on the realtime VCPU-1" is not acceptable.
>>
>> That shouldn't happen.  Sources of frequent interruptions have all been
>> fixed or moved outside the main thread.
>>
>> If there are more left, report the bug and we'll see how to fix it in
>> userspace.
> 
> What should not happen? The generation of 10.000 interrupts per second
> (say disk IO completion) on a given workload ?

If you know you have this kind disk workload, you must use virtio-blk or
virtio-scsi with iothreads and place the iothreads on their own physical
CPUs.

Among "run arbitrary workloads", "run real-time workloads", "pack stuff
into as few physical CPUs as possible", you can only pick two.

Paolo