linux-kernel - Re: [PATCH v9 2/4] vhost: Reintroduce kthread mode support in vhost

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Message-ID: <CACGkMEtCxn7rZfvo9_nUwC1TwqJ+5F2XDdU89rz=iyO3U0pCEQ@mail.gmail.com>
Date: Mon, 21 Apr 2025 11:39:14 +0800
From: Jason Wang <jasowang@...hat.com>
To: Cindy Lu <lulu@...hat.com>
Cc: mst@...hat.com, michael.christie@...cle.com, sgarzare@...hat.com, 
	linux-kernel@...r.kernel.org, virtualization@...ts.linux-foundation.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH v9 2/4] vhost: Reintroduce kthread mode support in vhost

On Mon, Apr 21, 2025 at 10:45 AM Cindy Lu <lulu@...hat.com> wrote:
>
> This patch reintroduces kthread mode support in vhost,
> It also introduces struct vhost_worker_ops to abstract
> worker create/stop/wakeup operations.
>
> * Bring back the original vhost_worker() implementation,
>   and renamed to vhost_run_work_kthread_list().
>
> * Add cgroup support for the kthread
>
> * Introduce struct vhost_worker_ops:
>   - Encapsulates create / stop / wake‑up callbacks.
>   - vhost_worker_create() selects the proper ops according to
>     inherit_owner.
>
> This partially reverts or improves upon:
> commit 6e890c5d5021 ("vhost: use vhost_tasks for worker threads")
> commit 1cdaafa1b8b4 ("vhost: replace single worker pointer with xarray")
>
> Signed-off-by: Cindy Lu <lulu@...hat.com>
> ---
>  drivers/vhost/vhost.c | 188 ++++++++++++++++++++++++++++++++++++++----
>  drivers/vhost/vhost.h |  12 +++
>  2 files changed, 182 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 250dc43f1786..be97028a8baf 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -22,6 +22,7 @@
>  #include <linux/slab.h>
>  #include <linux/vmalloc.h>
>  #include <linux/kthread.h>
> +#include <linux/cgroup.h>
>  #include <linux/module.h>
>  #include <linux/sort.h>
>  #include <linux/sched/mm.h>
> @@ -242,7 +243,7 @@ static void vhost_worker_queue(struct vhost_worker *worker,
>                  * test_and_set_bit() implies a memory barrier.
>                  */
>                 llist_add(&work->node, &worker->work_list);
> -               vhost_task_wake(worker->vtsk);
> +               worker->ops->wakeup(worker);
>         }
>  }
>
> @@ -388,6 +389,44 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>         __vhost_vq_meta_reset(vq);
>  }
>
> +static int vhost_run_work_kthread_list(void *data)
> +{
> +       struct vhost_worker *worker = data;
> +       struct vhost_work *work, *work_next;
> +       struct vhost_dev *dev = worker->dev;
> +       struct llist_node *node;
> +
> +       kthread_use_mm(dev->mm);
> +
> +       for (;;) {
> +               /* mb paired w/ kthread_stop */
> +               set_current_state(TASK_INTERRUPTIBLE);
> +
> +               if (kthread_should_stop()) {
> +                       __set_current_state(TASK_RUNNING);
> +                       break;
> +               }
> +               node = llist_del_all(&worker->work_list);
> +               if (!node)
> +                       schedule();
> +
> +               node = llist_reverse_order(node);
> +               /* make sure flag is seen after deletion */
> +               smp_wmb();
> +               llist_for_each_entry_safe(work, work_next, node, node) {
> +                       clear_bit(VHOST_WORK_QUEUED, &work->flags);
> +                       __set_current_state(TASK_RUNNING);
> +                       kcov_remote_start_common(worker->kcov_handle);
> +                       work->fn(work);
> +                       kcov_remote_stop();
> +                       cond_resched();
> +               }
> +       }
> +       kthread_unuse_mm(dev->mm);
> +
> +       return 0;
> +}
> +
>  static bool vhost_run_work_list(void *data)
>  {
>         struct vhost_worker *worker = data;
> @@ -582,6 +621,46 @@ long vhost_dev_check_owner(struct vhost_dev *dev)
>  }
>  EXPORT_SYMBOL_GPL(vhost_dev_check_owner);
>
> +struct vhost_attach_cgroups_struct {
> +       struct vhost_work work;
> +       struct task_struct *owner;
> +       int ret;
> +};
> +
> +static void vhost_attach_cgroups_work(struct vhost_work *work)
> +{
> +       struct vhost_attach_cgroups_struct *s;
> +
> +       s = container_of(work, struct vhost_attach_cgroups_struct, work);
> +       s->ret = cgroup_attach_task_all(s->owner, current);
> +}
> +
> +static int vhost_attach_task_to_cgroups(struct vhost_worker *worker)
> +{
> +       struct vhost_attach_cgroups_struct attach;
> +       int saved_cnt;
> +
> +       attach.owner = current;
> +
> +       vhost_work_init(&attach.work, vhost_attach_cgroups_work);
> +       vhost_worker_queue(worker, &attach.work);
> +
> +       mutex_lock(&worker->mutex);
> +
> +       /*
> +        * Bypass attachment_cnt check in __vhost_worker_flush:
> +        * Temporarily change it to INT_MAX to bypass the check
> +        */
> +       saved_cnt = worker->attachment_cnt;
> +       worker->attachment_cnt = INT_MAX;
> +       __vhost_worker_flush(worker);
> +       worker->attachment_cnt = saved_cnt;

I wonder if it's easier to re-introduce the flush that was used before
vhost kthread to avoid the tricks here. We can have flush ops for
example.

Thanks