Virtualization - Mar 2022 - [PATCH 1/2] vdpa: mlx5: prevent cvq work from hogging CPU

On Mon, 21 Mar 2022 17:00:09 +0800 Jason Wang wrote:
> 
> Ok, speak too fast.
Frankly I have fun running faster five days a week.
> So you meant to add a cond_resched() in the loop?
Yes, it is one liner.

Hillf

On Mon, Mar 21, 2022 at 8:34 PM Hillf Danton <hdanton at sina.com>
wrote:
>
> On Mon, 21 Mar 2022 17:00:09 +0800 Jason Wang wrote:
> >
> > Ok, speak too fast.
>
> Frankly I have fun running faster five days a week.
:)
>
> > So you meant to add a cond_resched() in the loop?
>
> Yes, it is one liner.
Yes, there will be no "infinite" loop, but since the loop is triggered
by userspace. It looks to me it will delay the flush/drain of the
workqueue forever which is still suboptimal.

Thanks
>
> Hillf
>

On Tue, 22 Mar 2022 09:59:14 +0800 Jason Wang wrote:
> 
> Yes, there will be no "infinite" loop, but since the loop is
triggered
> by userspace. It looks to me it will delay the flush/drain of the
> workqueue forever which is still suboptimal.
Usually it is barely possible to shoot two birds using a stone.

Given the "forever", I am inclined to not running faster, hehe, though
another cobble is to add another line in the loop checking if mvdev is
unregistered, and for example make mvdev->cvq unready before destroying
workqueue.

static void mlx5_vdpa_dev_del(struct vdpa_mgmt_dev *v_mdev, struct vdpa_device
*dev)
{
	struct mlx5_vdpa_mgmtdev *mgtdev = container_of(v_mdev, struct
mlx5_vdpa_mgmtdev, mgtdev);
	struct mlx5_vdpa_dev *mvdev = to_mvdev(dev);
	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);

	mlx5_notifier_unregister(mvdev->mdev, &ndev->nb);
	destroy_workqueue(mvdev->wq);
	_vdpa_unregister_device(dev);
	mgtdev->ndev = NULL;
}