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

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Message-ID: <951aaa3f-b20d-6f67-9454-f193f4445fc7@redhat.com>
Date:   Sat, 23 Sep 2017 12:56:12 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Marcelo Tosatti <mtosatti@...hat.com>
Cc:     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 22/09/2017 14:55, Peter Zijlstra wrote:
> You just explained it yourself. If the thread that needs to complete
> what you're waiting on has lower priority, it will _never_ get to run if
> you're busy waiting on it.
> 
> This is _trivial_.
> 
> And even for !RT it can be quite costly, because you can end up having
> to burn your entire slot of CPU time before you run the other task.
> 
> Userspace spinning is _bad_, do not do this.

This is not userspace spinning, it is guest spinning---which has
effectively the same effect but you cannot quite avoid.

But I agree that the solution is properly prioritizing threads that can
interrupt the VCPU, and using PI mutexes.

I'm not a priori opposed to paravirt scheduling primitives, but I am not
at all sure that it's required.

Paolo

> (the one exception where it works is where you have a single thread per
> cpu, because then there's effectively no scheduling).