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

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;
}

On Thu, Mar 24, 2022 at 8:54 AM Hillf Danton <hdanton at sina.com>
wrote:
>
> 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;
> }
>
Yes, so we had

1) using a quota for re-requeue
2) using something like

while (READ_ONCE(cvq->ready)) {
        ...
        cond_resched();
}

There should not be too much difference except we need to use
cancel_work_sync() instead of flush_work for 1).

I would keep the code as is but if you stick I can change.

Thanks

On Thu, 24 Mar 2022 10:34:09 +0800 Jason Wang wrote:
> On Thu, Mar 24, 2022 at 8:54 AM Hillf Danton <hdanton at sina.com>
wrote:
> >
> > 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;
> > }
> >
> 
> Yes, so we had
> 
> 1) using a quota for re-requeue
> 2) using something like
> 
> while (READ_ONCE(cvq->ready)) {
>         ...
>         cond_resched();
> }
> 
> There should not be too much difference except we need to use
> cancel_work_sync() instead of flush_work for 1).
> 
> I would keep the code as is but if you stick I can change.
No Sir I would not - I am simply not a fan of work requeue.

Hillf