linux-kernel - Re: [PATCH V3 6/9] virtio-ccw: implement synchronize

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Message-ID: <ba0c3977-c471-3275-2327-c5910cdd506a@redhat.com>
Date:   Tue, 26 Apr 2022 11:42:45 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Halil Pasic <pasic@...ux.ibm.com>
Cc:     Cornelia Huck <cohuck@...hat.com>, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, sgarzare@...hat.com,
        eperezma@...hat.com, lulu@...hat.com, tglx@...utronix.de,
        peterz@...radead.org, paulmck@...nel.org, maz@...nel.org
Subject: Re: [PATCH V3 6/9] virtio-ccw: implement synchronize_cbs()


在 2022/4/26 11:38, Michael S. Tsirkin 写道:
> On Mon, Apr 25, 2022 at 11:35:41PM -0400, Michael S. Tsirkin wrote:
>> On Tue, Apr 26, 2022 at 04:29:11AM +0200, Halil Pasic wrote:
>>> On Mon, 25 Apr 2022 09:59:55 -0400
>>> "Michael S. Tsirkin" <mst@...hat.com> wrote:
>>>
>>>> On Mon, Apr 25, 2022 at 10:54:24AM +0200, Cornelia Huck wrote:
>>>>> On Mon, Apr 25 2022, "Michael S. Tsirkin" <mst@...hat.com> wrote:
>>>>>    
>>>>>> On Mon, Apr 25, 2022 at 10:44:15AM +0800, Jason Wang wrote:
>>>>>>> This patch tries to implement the synchronize_cbs() for ccw. For the
>>>>>>> vring_interrupt() that is called via virtio_airq_handler(), the
>>>>>>> synchronization is simply done via the airq_info's lock. For the
>>>>>>> vring_interrupt() that is called via virtio_ccw_int_handler(), a per
>>>>>>> device spinlock for irq is introduced ans used in the synchronization
>>>>>>> method.
>>>>>>>
>>>>>>> Cc: Thomas Gleixner <tglx@...utronix.de>
>>>>>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>>>>>> Cc: "Paul E. McKenney" <paulmck@...nel.org>
>>>>>>> Cc: Marc Zyngier <maz@...nel.org>
>>>>>>> Cc: Halil Pasic <pasic@...ux.ibm.com>
>>>>>>> Cc: Cornelia Huck <cohuck@...hat.com>
>>>>>>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>>>>>>
>>>>>> This is the only one that is giving me pause. Halil, Cornelia,
>>>>>> should we be concerned about the performance impact here?
>>>>>> Any chance it can be tested?
>>>>> We can have a bunch of devices using the same airq structure, and the
>>>>> sync cb creates a choke point, same as registering/unregistering.
>>>> BTW can callbacks for multiple VQs run on multiple CPUs at the moment?
>>> I'm not sure I understand the question.
>>>
>>> I do think we can have multiple CPUs that are executing some portion of
>>> virtio_ccw_int_handler(). So I guess the answer is yes. Connie what do you think?
>>>
>>> On the other hand we could also end up serializing synchronize_cbs()
>>> calls for different devices if they happen to use the same airq_info. But
>>> this probably was not your question
>>
>> I am less concerned about  synchronize_cbs being slow and more about
>> the slowdown in interrupt processing itself.
>>
>>>> this patch serializes them on a spinlock.
>>>>
>>> Those could then pile up on the newly introduced spinlock.
>>>
>>> Regards,
>>> Halil
>> Hmm yea ... not good.
> Is there any other way to synchronize with all callbacks?


Maybe using rwlock as airq handler?

Thanks


>
>> -- 
>> MST