Re: [PATCH] task_work: remove fifo ordering guarantee

Eric Dumazet
2015-08-29T13:57:34Z

On Sat, 2015-08-29 at 14:49 +0200, Oleg Nesterov wrote:
> 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?

Well, I stated in the changelog we could. Obviously we can.

Adding 275 ms of pure overhead to perform this list reversal for files
to be closed is quite unfortunate.

> 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.

So it looks like there is no problem today, right, other than the
possibility to parse a long list while blocking IRQ ?

>
> Fifo just looks more sane to me.

Well, files are closed in a random order. These are the main user of
this stuff.

If this is that critical, maybe use 2 lists, one for stuff needing fifo,
and another one for un-ordered stuff (ed : file closing), and add a
boolean to task_work_add()/task_work_cancel(). This adds yet another
field into struct task_struct.

Now we also could question why we needed commit
4a9d4b024a3102fc083c925c242d98ac27b1c5f6 ("switch fput to task_work_add
") since it seems quite an overhead at task exit with 10^6 of files to
close.

I understood the 'schedule_work() for interrupt/kernel_thread callers'
part, but not the task_work_add() one.