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From: Mike Christie <michael.christie@oracle.com>
To: "Michael S. Tsirkin" <mst@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>
Cc: qemu-devel@nongnu.org, fam@euphon.net,
	linux-scsi@vger.kernel.org, target-devel@vger.kernel.org,
	jasowang@redhat.com, pbonzini@redhat.com,
	virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq
Date: Thu, 19 Nov 2020 16:11:40 +0000 (UTC)
Message-ID: <ceebdc90-3ffc-1563-ff85-12a848bcba18@oracle.com> (raw)
In-Reply-To: <20201119094315-mutt-send-email-mst@kernel.org>

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.

  reply index

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-12 23:18 Mike Christie
2020-11-12 23:19 ` [PATCH 1/1] qemu vhost scsi: add VHOST_SET_VRING_ENABLE support Mike Christie
2020-11-17 11:53   ` Stefan Hajnoczi
2020-12-02  9:59   ` Michael S. Tsirkin
2020-12-02 16:05     ` Michael Christie
2020-11-12 23:19 ` [PATCH 01/10] vhost: remove work arg from vhost_work_flush Mike Christie
2020-11-17 13:04   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 02/10] vhost scsi: remove extra flushes Mike Christie
2020-11-17 13:07   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 03/10] vhost poll: fix coding style Mike Christie
2020-11-17 13:07   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 04/10] vhost: support multiple worker threads Mike Christie
2020-11-12 23:19 ` [PATCH 05/10] vhost: poll support support multiple workers Mike Christie
2020-11-17 15:32   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 06/10] vhost scsi: make SCSI cmd completion per vq Mike Christie
2020-11-17 16:04   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 07/10] vhost, vhost-scsi: flush IO vqs then send TMF rsp Mike Christie
2020-11-17 16:05   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 08/10] vhost: move msg_handler to new ops struct Mike Christie
2020-11-17 16:08   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 09/10] vhost: add VHOST_SET_VRING_ENABLE support Mike Christie
2020-11-17 16:14   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 10/10] vhost-scsi: create a woker per IO vq Mike Christie
2020-11-17 16:40 ` [PATCH 00/10] vhost/qemu: thread per IO SCSI vq Stefan Hajnoczi
2020-11-17 19:13   ` Mike Christie
2020-11-18  9:54     ` Michael S. Tsirkin
2020-11-19 14:00       ` Stefan Hajnoczi
2020-11-18 11:31     ` Stefan Hajnoczi
2020-11-19 14:46       ` Michael S. Tsirkin
2020-11-19 16:11         ` Mike Christie [this message]
2020-11-19 16:24           ` Stefan Hajnoczi
2020-11-19 16:43             ` Mike Christie
2020-11-19 17:08               ` Stefan Hajnoczi
2020-11-20  8:45                 ` Stefan Hajnoczi
2020-11-20 12:31                   ` Michael S. Tsirkin
2020-12-01 12:59                     ` Stefan Hajnoczi
2020-12-01 13:45                       ` Stefano Garzarella
2020-12-01 17:43                         ` Stefan Hajnoczi
2020-12-02 10:35                           ` Stefano Garzarella
2020-11-23 15:17                   ` Stefano Garzarella
2020-11-18  5:17   ` Jason Wang
2020-11-18  6:57     ` Mike Christie
2020-11-18  7:19       ` Mike Christie
2020-11-18  7:54       ` Jason Wang
2020-11-18 20:06         ` Mike Christie
2020-11-19  4:35           ` Jason Wang
2020-11-19 15:49             ` Mike Christie

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