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

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Date: Wed, 13 Dec 2023 09:55:18 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Tobias Huschle <huschle@...ux.ibm.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 01:45:35PM +0100, Tobias Huschle wrote:
> 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.


So we are actually making an effort to be nice.
Documentation/kernel-hacking/hacking.rst says:

If you're doing longer computations: first think userspace. If you
**really** want to do it in kernel you should regularly check if you need
to give up the CPU (remember there is cooperative multitasking per CPU).
Idiom::

    cond_resched(); /* Will sleep */


and this is what vhost.c does.

At this point I'm not sure why it's appropriate to call schedule() as opposed to
cond_resched(). Ideas?


-- 
MST