Re: [PATCH v7 00/14] vhost: multiple worker support

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Hi Mike,

On Fri, Apr 28, 2023 at 11:35:20AM -0500, michael.christie@xxxxxxxxxx wrote:
> The following patches were built over Linux's tree. They allow us to
> support multiple vhost workers tasks per device. The design is a modified
> version of Stefan's original idea where userspace has the kernel create a
> worker and we pass back the pid. In this version instead of passing the
> pid between user/kernel space we use a worker_id which is just an integer
> managed by the vhost driver and we allow userspace to create and free
> workers and then attach them to virtqueues at setup time.
> All review comments from the past reviews should be handled. If I didn't
> reply to a review comment, I agreed with the comment and should have
> handled it in this posting. Let me know if I missed one.
> Results:
> --------
> fio jobs        1       2       4       8       12      16
> ----------------------------------------------------------
> 1 worker        160k   488k     -       -       -       -
> worker per vq   160k   310k    620k    1300k   1836k   2326k
> Notes:
> 0. This used a simple fio command:
> fio --filename=/dev/sdb  --direct=1 --rw=randrw --bs=4k \
> --ioengine=libaio --iodepth=128  --numjobs=$JOBS_ABOVE
> and I used a VM with 16 vCPUs and 16 virtqueues.
> 1. The patches were tested with LIO's emulate_pr=0 which drops the
> LIO PR lock use. This was a bottleneck at around 12 vqs/jobs.
> 2. Because we have a hard limit of 1024 cmds, if the num jobs * iodepth
> was greater than 1024, I would decrease iodepth. So 12 jobs used 85 cmds,
> and 16 used 64.
> 3. The perf issue above at 2 jobs is because when we only have 1 worker
> we execute more cmds per vhost_work due to all vqs funneling to one worker.
> I have 2 patches that fix this. One is just submit from the worker thread
> instead of kikcing off to a workqueue like how the vhost block patches do.
> I'll post this after the worker support is merged. I'm also working on
> patches to add back batching during lio completion and do polling on the
> submission side.
> We will still want the threading patches, because if we batch at the fio
> level plus use the vhost theading patches, we can see a big boost like
> below. So hopefully doing it at the kernel will allow apps to just work
> without having to be smart like fio.
> fio using io_uring and batching with the iodepth_batch* settings:
> fio jobs        1       2       4       8       12      16
> -------------------------------------------------------------
> 1 worker        494k    520k    -       -       -       -
> worker per vq   496k    878k    1542k   2436k   2304k   2590k

Could you rebase on latest rc and repost please?

> V7:
> - Add more comments about assumptions.
> - Drop refcounting and just use an attachment_cnt variable, so there
> is no confusion about when a worker is freed.
> - Do a opt-in model, because vsiock has an issue where it can queue works
> before it's running and it doesn't even need multiple workers, so there 
> is no point in chaning the driver or core code.
> - Add back get worker ioctl.
> - Broke up last patches to make it easier to read.
> V6:
> - Rebase against vhost_task patchset.
> - Used xa instead of idr.
> V5:
> - Rebase against user_worker patchset.
> - Rebase against flush patchset.
> - Redo vhost-scsi tmf flush handling so it doesn't access vq->worker.
> V4:
> - fix vhost-sock VSOCK_VQ_RX use.
> - name functions called directly by ioctl cmd's to match the ioctl cmd.
> - break up VHOST_SET_VRING_WORKER into a new, free and attach cmd.
> - document worker lifetime, and cgroup, namespace, mm, rlimit
> inheritance, make it clear we currently only support sharing within the
> device.
> - add support to attach workers while IO is running.
> - instead of passing a pid_t of the kernel thread, pass a int allocated
> by the vhost layer with an idr.
> V3:
> - fully convert vhost code to use vq based APIs instead of leaving it
> half per dev and half per vq.
> - rebase against kernel worker API.
> - Drop delayed worker creation. We always create the default worker at
> VHOST_SET_OWNER time. Userspace can create and bind workers after that.
> V2:
> - change loop that we take a refcount to the worker in
> - replaced pid == -1 with define.
> - fixed tabbing/spacing coding style issue
> - use hash instead of list to lookup workers.
> - I dropped the patch that added an ioctl cmd to get a vq's worker's
> pid. I saw we might do a generic netlink interface instead.

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