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

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Message-ID: <ZXmnb85uoOzDkoYy@DESKTOP-2CCOB1S.>
Date: Wed, 13 Dec 2023 13:45:35 +0100
From: Tobias Huschle <huschle@...ux.ibm.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>, Abel Wu <wuyun.abel@...edance.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        virtualization@...ts.linux.dev, netdev@...r.kernel.org
Subject: Re: Re: Re: EEVDF/vhost regression (bisected to 86bfbb7ce4f6
 sched/fair: Add lag based placement)

On Wed, Dec 13, 2023 at 07:00:53AM -0500, Michael S. Tsirkin wrote:
> On Wed, Dec 13, 2023 at 11:37:23AM +0100, Tobias Huschle wrote:
> > On Tue, Dec 12, 2023 at 11:15:01AM -0500, Michael S. Tsirkin wrote:
> > > On Tue, Dec 12, 2023 at 11:00:12AM +0800, Jason Wang wrote:
> > > > On Tue, Dec 12, 2023 at 12:54 AM Michael S. Tsirkin <mst@...hat.com> wrote:

[...]
> 
> 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.