Re: [PATCH] task_work: remove fifo ordering guarantee

[Oleg Nesterov <oleg@redhat.com>]

On 08/31, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@redhat.com> wrote:
>
> > On 08/29, Ingo Molnar wrote:
> > >
> > > So I'm wondering, is there any strong reason why we couldn't use a double linked
> > > list and still do FIFO and remove that silly linear list walking hack?
> >
> > This will obviously enlarge callback_head, and it is often embedded.
> > But this is minor.
> >
> > If we use a double linked list we can't do task_work_add() lockless.
> > So we will need another spinlock_t in task_struct. We can't use pi_lock.
>
> The fact that the O(N) overhead was measured in real apps to be in the
> milliseconds IMHO weakens cycle-level concerns about also having a spinlock next
> to the list head. (There's no additional cacheline bouncing concerns with the
> spinlock: the head of a LIFO list is essentially a bouncing cacheline.)

I agree. I just tried to explain that we need a bit more changes than
just s/callback_head/list_head/ in task_struct.

And. The fact that this O(N) overhead was measured means that we have
more overhead with offload-fput-to-exit_task_work which would be nice
to remove as well.

Oleg.