Re: [PATCH] task_work: remove fifo ordering guarantee

Oleg Nesterov
2015-08-29T12:51:57Z

On 08/28, Eric Dumazet wrote:
>
> From: Eric Dumazet <edumazet@google.com>
>
> In commit f341861fb0b ("task_work: add a scheduling point in
> task_work_run()") I fixed a latency problem adding a cond_resched()
> call.
>
> Later, commit ac3d0da8f329 added yet another loop to reverse a list,
> bringing back the latency spike :
>
> I've seen in some cases this loop taking 275 ms, if for example a
> process with 2,000,000 files is killed.
>
> We could add yet another cond_resched() in the reverse loop,

Can't we do this?

> or we
> can simply remove the reversal, as I do not think anything
> would depend on order of task_work_add() submitted works.

Personally I'd prefer to keep the fifo ordering. It just makes
more sense imho. Even if currently nobody depends on it (although
I am not sure about out-of-tree modules, say, systemtap).

Let's look keyctl_session_to_parent(). It does task_work_cancel()
but only because we can not trust user-space. Otherwise we could
remove it and just do task_work_add(), but this needs fifo.

Fifo just looks more sane to me.

Oleg.