Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq

[Mike Christie <michael.christie@oracle.com>]

On 11/19/20 10:24 AM, Stefan Hajnoczi wrote:
> On Thu, Nov 19, 2020 at 4:13 PM Mike Christie
> <michael.christie@oracle.com> wrote:
>>
>> On 11/19/20 8:46 AM, Michael S. Tsirkin wrote:
>>> On Wed, Nov 18, 2020 at 11:31:17AM +0000, Stefan Hajnoczi wrote:
>>>>> My preference has been:
>>>>>
>>>>> 1. If we were to ditch cgroups, then add a new interface that would allow
>>>>> us to bind threads to a specific CPU, so that it lines up with the guest's
>>>>> mq to CPU mapping.
>>>>
>>>> A 1:1 vCPU/vq->CPU mapping isn't desirable in all cases.
>>>>
>>>> The CPU affinity is a userspace policy decision. The host kernel should
>>>> provide a mechanism but not the policy. That way userspace can decide
>>>> which workers are shared by multiple vqs and on which physical CPUs they
>>>> should run.
>>>
>>> So if we let userspace dictate the threading policy then I think binding
>>> vqs to userspace threads and running there makes the most sense,
>>> no need to create the threads.
>>>
>>
>> Just to make sure I am on the same page, in one of the first postings of
>> this set at the bottom of the mail:
>>
>> https://urldefense.com/v3/__https://www.spinics.net/lists/linux-scsi/msg148322.html__;!!GqivPVa7Brio!PdGIFdzqcAb6DW8twtjX3r7xOcM7XbTh7Ndkhxhb-1fV1VNB4lXjFzwFVE1zczUIE2Mp$
>>
>> I asked about a new interface and had done something more like what
>> Stefan posted:
>>
>>     struct vhost_vq_worker_info {
>>         /*
>>          * The pid of an existing vhost worker that this vq will be
>>          * assigned to. When pid is 0 the virtqueue is assigned to the
>>          * default vhost worker. When pid is -1 a new worker thread is
>>          * created for this virtqueue. When pid is -2 the virtqueue's
>>          * worker thread is unchanged.
>>          *
>>          * If a vhost worker no longer has any virtqueues assigned to it
>>          * then it will terminate.
>>          *
>>          * The pid of the vhost worker is stored to this field when the
>>          * ioctl completes successfully. Use pid -2 to query the current
>>          * vhost worker pid.
>>          */
>>         __kernel_pid_t pid;  /* in/out */
>>
>>         /* The virtqueue index*/
>>         unsigned int vq_idx; /* in */
>>     };
>>
>> This approach is simple and it allowed me to have userspace map queues
>> and threads optimally for our setups.
>>
>> Note: Stefan, in response to your previous comment, I am just using my
>> 1:1 mapping as an example and would make it configurable from userspace.
>>
>> In the email above are you guys suggesting to execute the SCSI/vhost
>> requests in userspace? We should not do that because:
>>
>> 1. It negates part of what makes vhost fast where we do not have to kick
>> out to userspace then back to the kernel.
>>
>> 2. It's not doable or becomes a crazy mess because vhost-scsi is tied to
>> the scsi/target layer in the kernel. You can't process the scsi command
>> in userspace since the scsi state machine and all its configuration info
>> is in the kernel's scsi/target layer.
>>
>> For example, I was just the maintainer of the target_core_user module
>> that hooks into LIO/target on the backend (vhost-scsi hooks in on the
>> front end) and passes commands to userspace and there we have a
>> semi-shadow state machine. It gets nasty to try and maintain/sync state
>> between lio/target core in the kernel and in userspace. We also see the
>> perf loss I mentioned in #1.
> 
> No, if I understand Michael correctly he has suggested a different approach.
> 
> My suggestion was that the kernel continues to manage the worker
> threads but an ioctl allows userspace to control the policy.
> 
> I think Michael is saying that the kernel shouldn't manage/create
> threads. Userspace should create threads and then invoke an ioctl from
> those threads.
> 
> The ioctl will call into the vhost driver where it will execute
> something similar to vhost_worker(). So this ioctl will block while
> the kernel is using the thread to process vqs.
> 
> What isn't clear to me is how to tell the kernel which vqs are
> processed by a thread. We could try to pass that information into the
> ioctl. I'm not sure what the cleanest solution is here.
> 
> Maybe something like:
> 
> struct vhost_run_worker_info {
>      struct timespec *timeout;
>      sigset_t *sigmask;
> 
>      /* List of virtqueues to process */
>      unsigned nvqs;
>      unsigned vqs[];
> };
> 
> /* This blocks until the timeout is reached, a signal is received, or
> the vhost device is destroyed */
> int ret = ioctl(vhost_fd, VHOST_RUN_WORKER, &info);
> 
> As you can see, userspace isn't involved with dealing with the
> requests. It just acts as a thread donor to the vhost driver.
> 
> We would want the VHOST_RUN_WORKER calls to be infrequent to avoid the
> penalty of switching into the kernel, copying in the arguments, etc.

I didn't get this part. Why have the timeout? When the timeout expires, 
does userspace just call right back down to the kernel or does it do 
some sort of processing/operation?

You could have your worker function run from that ioctl wait for a 
signal or a wake up call from the vhost_work/poll functions.


> 
> Michael: is this the kind of thing you were thinking of?
> 
> Stefan
>