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

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

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://www.spinics.net/lists/linux-scsi/msg148322.html

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.