linux-kernel - Re: [RFC][PATCH 0/4] arm64:kvm: teach guest sched that VCPUs can be preempted

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Message-ID: <20200817020310.GA1210848@jagdpanzerIV.localdomain>
Date:   Mon, 17 Aug 2020 11:03:10 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>
Cc:     James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        suleiman@...gle.com, joelaf@...gle.com,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC][PATCH 0/4] arm64:kvm: teach guest sched that VCPUs can be
 preempted

On (20/07/21 13:17), Sergey Senozhatsky wrote:
> Hello,
> 
> 	RFC
> 
> 	We noticed that in a number of cases when we wake_up_process()
> on arm64 guest we end up enqueuing that task on a preempted VCPU. The culprit
> appears to be the fact that arm64 guests are not aware of VCPU preemption
> as such, so when sched picks up an idle VCPU it always assumes that VCPU
> is available:
> 
>       wake_up_process()
>        try_to_wake_up()
>         select_task_rq_fair()
>          available_idle_cpu()
>           vcpu_is_preempted()    // return false;
> 
> Which is, obviously, not the case.
> 
> This RFC patch set adds a simple vcpu_is_preempted() implementation so
> that scheduler can make better decisions when it search for the idle
> (v)CPU.

Hi,

A gentle ping.

	-ss