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

[yezengruan <yezengruan@huawei.com>]

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@lists.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