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

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Date:   Mon, 2 Oct 2017 14:30:33 +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 29/09/2017 22:17, Marcelo Tosatti wrote:
> On Fri, Sep 29, 2017 at 07:05:41PM +0200, Paolo Bonzini wrote:
>> On 29/09/2017 18:40, Marcelo Tosatti wrote:
>>> Thats not the state of things (userspace in vcpu-0 is not specially tailored
>>> to not violate latencies in vcpu-1): that is not all user triggered
>>> actions can be verified.
>>>
>>> Think "updatedb", and so on...
>>
>> _Which_ spinlock is it that can cause unwanted latency while running
>> updatedb on VCPU0 and a real-time workload on VCPU1, and only so on virt
>> because of the emulator thread?
> 
> Hundreds of them (the one being hit is in timer_interrupt), but i went 
> to check and there are hundreds of raw spinlocks shared between the
> kernel threads that run on isolated CPUs and vcpu-0.
> 
>>  Is this still broken if you set up
>> priorities for the emulator thread correctly and use PI mutexes in QEMU?
> 
> I don't see why it would not, if you have to schedule the emulator
> thread to process and inject I/O interrupts for example.

Yes, you're right if it's interrupt injections.  If it's unexpected disk
accesses, you can just add a QEMU I/O thread on a different physical
CPU.  The same physical CPU can host I/O threads for different guests if
you expect them to do little.

I don't understand why is it correct to delay interrupt injection just
because VCPU0 is running in a spinlock-protected region?  I just cannot
see the reason why it's safe and not a recipe for priority inversions.

Paolo

>>  And if so, what is the cause of interruptions in the emulator thread
>> and how are these interruptions causing the jitter?
> 
> Interrupt injections.
> 
>> Priorities and priority inheritance (or lack of them) is a _known_
>> issue.  Jan was doing his KVM-RT things in 2009 and he was talking about
>> priorities[1] back then.  The effect of correct priorities is to _lower_
>> jitter, not to make it worse, and anyway certainly not worse than
>> SCHED_NORMAL I/O thread.  Once that's fixed, we can look at other problems.
>>
>> Paolo
>>
>> [1] http://static.lwn.net/images/conf/rtlws11/papers/proc/p18.pdf which
>> also mentions pv scheduling