Linux Virtualization - Nov 2014 - [PATCH v3] virtio_balloon: Convert "vballoon" kthread into a workqueue

On Thu, Nov 20, 2014 at 06:26:24PM +0200, Michael S. Tsirkin
wrote:
> On Thu, Nov 20, 2014 at 06:25:43PM +0200, Michael S. Tsirkin wrote:
> > On Thu, Nov 20, 2014 at 11:07:46AM -0500, Tejun Heo wrote:
> > > On Thu, Nov 20, 2014 at 05:03:17PM +0100, Petr Mladek wrote:
> > > ...
> > > > @@ -476,7 +460,6 @@ static void virtballoon_remove(struct
virtio_device *vdev)
> > > >  {
> > > >  	struct virtio_balloon *vb = vdev->priv;
> > > >  
> > > > -	kthread_stop(vb->thread);
> > > >  	remove_common(vb);
> > > >  	kfree(vb);
> > > >  }
> > > 
> > > Shouldn't the work item be flushed before removal is
complete?
> > 
> > In fact, flushing it won't help because it can requeue itself,
right?
There's cancel_work_sync() to stop the self-requeueing ones.
> From that POV a dedicated WQ kept it simple.
A dedicated wq doesn't do anything for that.  You can't shut down a
workqueue with a pending work item on it.  destroy_workqueue() will
try to drain the target wq, warn if it doesn't finish in certain
number of iterations and just keep trying indefinitely.

Thanks.

-- 
tejun

On Thu, Nov 20, 2014 at 11:29:35AM -0500, Tejun Heo
wrote:
> On Thu, Nov 20, 2014 at 06:26:24PM +0200, Michael S. Tsirkin wrote:
> > On Thu, Nov 20, 2014 at 06:25:43PM +0200, Michael S. Tsirkin wrote:
> > > On Thu, Nov 20, 2014 at 11:07:46AM -0500, Tejun Heo wrote:
> > > > On Thu, Nov 20, 2014 at 05:03:17PM +0100, Petr Mladek wrote:
> > > > ...
> > > > > @@ -476,7 +460,6 @@ static void
virtballoon_remove(struct virtio_device *vdev)
> > > > >  {
> > > > >  	struct virtio_balloon *vb = vdev->priv;
> > > > >  
> > > > > -	kthread_stop(vb->thread);
> > > > >  	remove_common(vb);
> > > > >  	kfree(vb);
> > > > >  }
> > > > 
> > > > Shouldn't the work item be flushed before removal is
complete?
> > > 
> > > In fact, flushing it won't help because it can requeue
itself, right?
> 
> There's cancel_work_sync() to stop the self-requeueing ones.
What happens if queue_work runs while cancel_work_sync is in progress?
Does it fail to queue?
> > From that POV a dedicated WQ kept it simple.
> 
> A dedicated wq doesn't do anything for that.  You can't shut down a
> workqueue with a pending work item on it.  destroy_workqueue() will
> try to drain the target wq, warn if it doesn't finish in certain
> number of iterations and just keep trying indefinitely.
> 
> Thanks.
Right, so eventually we'll stop requeueuing and it will succeed?
> -- 
> tejun

On Thu, Nov 20, 2014 at 06:47:11PM +0200, Michael S. Tsirkin
wrote:
> > There's cancel_work_sync() to stop the self-requeueing ones.
> 
> What happens if queue_work runs while cancel_work_sync is in progress?
> Does it fail to queue?
cancel_work_sync() is guaranteed to take self-requeueing work items no
matter when it's called or what's going on.  External (non-self)
queueings of course should be stopped in other ways.
> > > From that POV a dedicated WQ kept it simple.
> > 
> > A dedicated wq doesn't do anything for that.  You can't shut
down a
> > workqueue with a pending work item on it.  destroy_workqueue() will
> > try to drain the target wq, warn if it doesn't finish in certain
> > number of iterations and just keep trying indefinitely.
> > 
> > Thanks.
> 
> Right, so eventually we'll stop requeueuing and it will succeed?
Yeah, sure, it's a silly reason to use a separate workqueue tho.
Don't do it that way.

Thanks.

-- 
tejun

On Thu 2014-11-20 11:29:35, Tejun Heo wrote:
> On Thu, Nov 20, 2014 at 06:26:24PM +0200, Michael S. Tsirkin wrote:
> > On Thu, Nov 20, 2014 at 06:25:43PM +0200, Michael S. Tsirkin wrote:
> > > On Thu, Nov 20, 2014 at 11:07:46AM -0500, Tejun Heo wrote:
> > > > On Thu, Nov 20, 2014 at 05:03:17PM +0100, Petr Mladek wrote:
> > > > ...
> > > > > @@ -476,7 +460,6 @@ static void
virtballoon_remove(struct virtio_device *vdev)
> > > > >  {
> > > > >  	struct virtio_balloon *vb = vdev->priv;
> > > > >  
> > > > > -	kthread_stop(vb->thread);
> > > > >  	remove_common(vb);
> > > > >  	kfree(vb);
> > > > >  }
> > > > 
> > > > Shouldn't the work item be flushed before removal is
complete?
Great catch!
> > > In fact, flushing it won't help because it can requeue
itself, right?
> 
> There's cancel_work_sync() to stop the self-requeueing ones.
Ah, one more problem is that remove_common(vb) calls leak_balloon()
that queues the work if not finished. We would need to add some flag
or variant that would disable the queuing when called here.

> > From that POV a dedicated WQ kept it simple.
> 
> A dedicated wq doesn't do anything for that.  You can't shut down a
> workqueue with a pending work item on it.  destroy_workqueue() will
> try to drain the target wq, warn if it doesn't finish in certain
> number of iterations and just keep trying indefinitely.
I wonder if it is guaranteed that none would trigger
stats_request() or virtballoon_changed() when virtballoon_remove() is
being called. I guess so because the original code would fail
otherwise. The two functions access "vb->config_change"
and the structure is freed in virtballoon_remove() without
any protection.

I am trying to confirm this by reading the code but it is not that
easy.

Best Regards,
Petr

On Thu, Nov 20, 2014 at 05:55:58PM +0100, Petr Mladek
wrote:
> On Thu 2014-11-20 11:29:35, Tejun Heo wrote:
> > On Thu, Nov 20, 2014 at 06:26:24PM +0200, Michael S. Tsirkin wrote:
> > > On Thu, Nov 20, 2014 at 06:25:43PM +0200, Michael S. Tsirkin
wrote:
> > > > On Thu, Nov 20, 2014 at 11:07:46AM -0500, Tejun Heo wrote:
> > > > > On Thu, Nov 20, 2014 at 05:03:17PM +0100, Petr Mladek
wrote:
> > > > > ...
> > > > > > @@ -476,7 +460,6 @@ static void
virtballoon_remove(struct virtio_device *vdev)
> > > > > >  {
> > > > > >  	struct virtio_balloon *vb = vdev->priv;
> > > > > >  
> > > > > > -	kthread_stop(vb->thread);
> > > > > >  	remove_common(vb);
> > > > > >  	kfree(vb);
> > > > > >  }
> > > > > 
> > > > > Shouldn't the work item be flushed before removal
is complete?
> 
> Great catch!
> 
> > > > In fact, flushing it won't help because it can requeue
itself, right?
> > 
> > There's cancel_work_sync() to stop the self-requeueing ones.
> 
> Ah, one more problem is that remove_common(vb) calls leak_balloon()
> that queues the work if not finished. We would need to add some flag
> or variant that would disable the queuing when called here.
> 
That's why Tejun suggested cancel_work_sync, IIUC it stops
the requeuing without need for extra flags.
> > > From that POV a dedicated WQ kept it simple.
> > 
> > A dedicated wq doesn't do anything for that.  You can't shut
down a
> > workqueue with a pending work item on it.  destroy_workqueue() will
> > try to drain the target wq, warn if it doesn't finish in certain
> > number of iterations and just keep trying indefinitely.
> 
> I wonder if it is guaranteed that none would trigger
> stats_request() or virtballoon_changed() when virtballoon_remove() is
> being called. I guess so because the original code would fail
> otherwise. The two functions access "vb->config_change"
> and the structure is freed in virtballoon_remove() without
> any protection.
> 
> I am trying to confirm this by reading the code but it is not that
> easy.
> 
> Best Regards,
> Petr
It's synchronized through hardware.  remove_common calls reset and
del_vqs which will prevent new interrupts.

-- 
MST