linux-kernel - Re: Regression on vcpu_is

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Message-ID: <d00f7b30-4b34-3d3d-98c5-d0db1a5d9c4f@huawei.com>
Date:   Sat, 29 Oct 2022 17:15:15 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 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?

Thanks,
Miaohe Lin