Re: [PATCH] task_work: remove fifo ordering guarantee

Ingo Molnar
2015-08-31T06:02:14Z

* 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.)

If there's some other solution, sure, but LIFO queues tend to be trouble down the
line.

Thanks,

Ingo