Re: [PATCH 7/9] vhost: allow userspace to create workers

[Stefan Hajnoczi <stefanha@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 3439 bytes --]

On Sat, Jun 05, 2021 at 06:53:58PM -0500, michael.christie@oracle.com wrote:
> On 6/3/21 9:30 AM, Stefan Hajnoczi wrote:
> >> +	if (info->pid == VHOST_VRING_NEW_WORKER) {
> >> +		worker = vhost_worker_create(dev);
> > 
> > The maximum number of kthreads created is limited by
> > vhost_dev_init(nvqs)? For example VHOST_SCSI_MAX_VQ 128.
> > 
> > IIUC kthread_create is not limited by or accounted against the current
> > task, so I'm a little worried that a process can create a lot of
> > kthreads.
> > 
> > I haven't investigated other kthread_create() users reachable from
> > userspace applications to see how they bound the number of threads
> > effectively.
> 
> Do we want something like io_uring's copy_process use? It's what fork uses,
> so we get checks like RLIMIT_NPROC and max_threads.
> 
> I know I didn't look at everything, but it looks like for some software
> drivers we just allow the user to run wild. For example for nbd, when we
> create the device to do alloc_workqueue and use the default max_active
> value (256). We then don't have a limit on connections, so we could end
> up with 256 workqueue threads per device. And then there is no limit on
> devices a user can make.
> 
> 
> > 
> > Any thoughts?
> >
> 
> Is the concern a bad VM could create N devs each with 128 vqs/threads
> and it would slow down other VMs? How do we handle the case where
> some VM makes M * N devs and that is equal to N * 128 so we would end
> up with the same number of threads either way? Is there a limit to the
> number of vhost devices a VM can make and can I just stick in a similar
> check for workers?
> 
> For vhost-scsi specifically, the 128 limit does not make a lot of sense.
> I think we want the max to be the number of vCPUs the VM has so we can
> add checks for that. Then we would assume someone making a VM with lots of
> CPUs is going to have the resources to support them.
> 
> Note: It does make sense from the point of view that we don't know the
> number of vCPUs when vhost-scsi calls vhost_dev_init, so I get we had to
> select an initial limit.

My concern is that threads should probably accounted against
RLIMIT_NPROC and max_threads rather than something indirect like 128 *
RLIMIT_NOFILE (a userspace process can only have RLIMIT_NOFILE
vhost-user file descriptors open).

> >> +		if (!dev->workers) {
> >> +			vhost_worker_put(worker);
> >> +			return -ENOMEM;
> >> +		}
> >> +	}
> >> +
> >> +	vq->worker = worker;
> >> +
> >> +	dev->workers[dev->num_workers] = worker;
> >> +	dev->num_workers++;
> > 
> > Hmm...should we really append to workers[] in the vhost_worker_find()
> > case?
> 
> 
> As it's coded now, yes. Every successful vhost_worker_find call does a
> get on the worker's refcount. Later when we delete the device, we loop
> over the workers array and for every entry we do a put.
> 
> I can add in some code to first check if the worker is already in the
> dev's worker list. If so then skip the refcount and skip adding to the
> workers array. If not in the dev's worker list then do a vhost_worker_find.
> 
> I thought it might be nicer how it is now with the single path. It's less
> code at least. Later if we add support to change a vq's worker then we also
> don't have to worry about refcounts as much. We just always drop the count
> taken from when it was added.

Thanks for explaining.

Stefan

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]