linux-kernel - Re: Regression on vcpu_is

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Message-ID: <Y10bPHJh7g+4OW4k@hirez.programming.kicks-ass.net>
Date:   Sat, 29 Oct 2022 14:23:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     "mingo@...hat.com" <mingo@...hat.com>, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, rohit.k.jain@...cle.com,
        dietmar.eggemann@....com, Steven Rostedt <rostedt@...dmis.org>,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Regression on vcpu_is_preempted()

On Sat, Oct 29, 2022 at 05:15:15PM +0800, Miaohe Lin wrote:
> On 2022/10/29 16:58, Peter Zijlstra wrote:
> > On Fri, Oct 28, 2022 at 04:48:21PM +0800, Miaohe Lin wrote:
> >>   When scheduler tries to select a CPU to run the gc thread,
> >>   available_idle_cpu() will check whether vcpu_is_preempted().  It
> >>   will choose other vcpu to run gc threads when the current vcpu is
> >>   preempted. But the preempted vcpu has no other work to do except
> >>   continuing to do gc. In our guest, there are more vcpus than java gc
> >>   threads. So there could always be some available vcpus when
> >>   scheduler tries to select a idle vcpu (runing on host). This leads
> >>   to lots of cpu migrations and results in regression.
> >>
> >>   I'm not really familiar with this mechanism. Is this a problem that
> >>   needs to be fixed or improved? Or is this just expected behavior?
> >>   Any response would be really appreciated!
> > 
> > This is pretty much expected behaviour. When a vCPU is preempted the
> > guest cannot know it's state or latency. Typically in the overcomitted
> > case another vCPU will be running on the CPU and getting our vCPU thread
> > back will take a considerable amount of time.
> 
> I see. Many thanks for your kindly reply and explanation. :)
> 
> > 
> > If you know you're not over-committed, perhaps you should configure your
> > VM differently.
> 
> Do you have any suggestion about how should I configure my VM when it's not over-committed?

I'm not an expert on VMs, but IIRC when you construct a pinned VM (ie.
1:1 vCPU:CPU relations) this all goes away.