Re: Re: Re: EEVDF/vhost regression (bisected to 86bfbb7ce4f6 sched/fair: Add lag based placement)

On Wed, Dec 13, 2023 at 01:45:35PM +0100, Tobias Huschle wrote:

On Wed, Dec 13, 2023 at 07:00:53AM -0500, Michael S. Tsirkin wrote:

quoted

On Wed, Dec 13, 2023 at 11:37:23AM +0100, Tobias Huschle wrote:

quoted

On Tue, Dec 12, 2023 at 11:15:01AM -0500, Michael S. Tsirkin wrote:

quoted

On Tue, Dec 12, 2023 at 11:00:12AM +0800, Jason Wang wrote:

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On Tue, Dec 12, 2023 at 12:54 AM Michael S. Tsirkin [off-list ref] wrote:

[...]

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Apparently schedule is already called?

What about this: 

static int vhost_task_fn(void *data)
{
	<...>
	did_work = vtsk->fn(vtsk->data);  --> this calls vhost_worker if I'm not mistaken
	if (!did_work)
		schedule();
	<...>
}

static bool vhost_worker(void *data)
{
	struct vhost_worker *worker = data;
	struct vhost_work *work, *work_next;
	struct llist_node *node;

	node = llist_del_all(&worker->work_list);
	if (node) {
		<...>
		llist_for_each_entry_safe(work, work_next, node, node) {
			<...>
		}
	}

	return !!node;
}

The llist_for_each_entry_safe does not actually change the node value, doesn't it?

If it does not change it, !!node would return 1.
Thereby skipping the schedule.

This was changed recently with:
f9010dbdce91 fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

It returned a hardcoded 0 before. The commit message explicitly mentions this
change to make vhost_worker return 1 if it did something.

Seems indeed like a nasty little side effect caused by EEVDF not scheduling
the woken up kworker right away.

Indeed, but previously vhost_worker was looping itself.
And it did:
-               node = llist_del_all(&worker->work_list);
-               if (!node)
-                       schedule();

so I don't think this was changed at all.






-- 
MST