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

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Date:   Mon, 17 Aug 2020 13:25:05 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     yezengruan <yezengruan@...wei.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>, will@...nel.org,
        joelaf@...gle.com, linux-kernel@...r.kernel.org,
        suleiman@...gle.com, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org,
        "Wanghaibin (D)" <wanghaibin.wang@...wei.com>
Subject: Re: [RFC][PATCH 0/4] arm64:kvm: teach guest sched that VCPUs can be
 preempted

On 2020-08-17 13:03, yezengruan wrote:
> On 2020/8/17 10:03, Sergey Senozhatsky wrote:
>> 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
>> _______________________________________________
>> kvmarm mailing list
>> kvmarm@...ts.cs.columbia.edu
>> https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
>> .
> 
> Hi Sergey,
> 
> I have a set of patches similar to yours.
> 
> https://lore.kernel.org/lkml/20191226135833.1052-1-yezengruan@huawei.com/

It really isn't the same thing at all. You are exposing PV spinlocks,
while Sergey exposes preemption to vcpus. The former is a massive,
and probably unnecessary superset of the later, which only impacts
the scheduler (it doesn't change the way locks are implemented).

You really shouldn't conflate the two (which you have done in your
series).

         M.
-- 
Jazz is not dead. It just smells funny...

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