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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 14 Dec 2023 02:14:59 -0500

On Wed, Dec 13, 2023 at 09:55:23AM -0500, Michael S. Tsirkin wrote:
> 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 <[email-masked]> 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?
> 

Peter, would appreciate feedback on this. When is cond_resched()
insufficient to give up the CPU? Should Documentation/kernel-hacking/hacking.rst
be updated to require schedule() instead?

> -- 
> MST