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From:   Jason Wang <jasowang@redhat.com>
Date:   Thu, 24 Mar 2022 10:34:09 +0800
Message-ID: <CACGkMEt7i3aA+qnsBjyvoZLFBsmbg3vDvvjd_N2MnJeJ8DuY4g@mail.gmail.com>
Subject: Re: [PATCH 1/2] vdpa: mlx5: prevent cvq work from hogging CPU
To:     Hillf Danton <hdanton@sina.com>
Cc:     mst <mst@redhat.com>, Eli Cohen <elic@nvidia.com>,
        virtualization <virtualization@lists.linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Thu, Mar 24, 2022 at 8:54 AM Hillf Danton <hdanton@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