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

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Message-ID: <3ff3b016-3f63-7d03-ed4b-c98d74db4af8@huawei.com>
Date:   Mon, 17 Aug 2020 22:15:39 +0800
From:   yezengruan <yezengruan@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "will@...nel.org" <will@...nel.org>, <joelaf@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        <suleiman@...gle.com>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "Wanghaibin (D)" <wanghaibin.wang@...wei.com>,
        <yezengruan@...wei.com>
Subject: Re: [RFC][PATCH 0/4] arm64:kvm: teach guest sched that VCPUs can be
 preempted

On 2020/8/17 20:25, Marc Zyngier wrote:
> 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.

Hi Marc,

Actually, both series support paravirtualization vcpu_is_preempted. My
series regard this as PV lock, but only the vcpu_is_preempted interface
of pv_lock_opt is implemented.

Except wake_up_process(), the vcpu_is_preempted interface of the current
kernel is used in the following scenarios:

kernel/sched/core.c:                            <---- wake_up_process()
--------------------
available_idle_cpu
    vcpu_is_preempted

kernel/locking/rwsem.c:
-----------------------
rwsem_optimistic_spin
    rwsem_spin_on_owner
        owner_on_cpu
            vcpu_is_preempted

kernel/locking/mutex.c:
-----------------------
mutex_optimistic_spin
    mutex_spin_on_owner
        vcpu_is_preempted

kernel/locking/osq_lock.c:
--------------------------
osq_lock
    vcpu_is_preempted

Thanks,

Zengruan

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