Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

Uladzislau Rezki
2020-09-21T19:48:26Z

Hello, Michal.

> >
> > Yes, I do well remember that you are unhappy with this approach.
> > Unfortunately, thus far, there is no solution that makes all developers
> > happy. You might be glad to hear that we are also looking into other
> > solutions, each of which makes some other developers unhappy. So we
> > are at least not picking on you alone. :-/
>
> No worries I do not feel like a whipping boy here. But do expect me to
> argue against the approach. I would also appreciate it if there was some
> more information on other attempts, why they have failed. E.g. why
> pre-allocation is not an option that works well enough in most
> reasonable workloads.
Pre-allocating has some drawbacks:

a) It is impossible to predict how many pages will be required to
cover a demand that is controlled by different workloads on
various systems.

b) Memory overhead since we do not know how much pages should be
preloaded: 100, 200 or 300

As for memory overhead, it is important to reduce it because of
embedded devices like phones, where a low memory condition is a
big issue. In that sense pre-allocating is something that we strongly
would like to avoid.

>
> I would also appreciate some more thoughts why we
> need to optimize for heavy abusers of RCU (like close(open) extremes).
>
I think here is a small misunderstanding. Please note, that is not only
about performance and corner cases. There is a single argument support
of the kvfree_rcu(ptr), where maintaining an array in time is needed.
The fallback of the single argument case is extrimely slow.

Single-argument details is here: https://lkml.org/lkml/2020/4/28/1626

> > > I strongly agree with Thomas http://lkml.kernel.org/r/87tux4kefm.fsf@nanos.tec.linutronix.de
> > > that this optimization is not aiming at reasonable workloads. Really, go
> > > with pre-allocated buffer and fallback to whatever slow path you have
> > > already. Exposing more internals of the allocator is not going to do any
> > > good for long term maintainability.
> >
> > I suggest that you carefully re-read the thread following that email.
>
> I clearly remember Thomas not being particularly happy that you optimize
> for a corner case. I do not remember there being a consensus that this
> is the right approach. There was some consensus that this is better than
> a gfp flag. Still quite bad though if you ask me.
>
> > Given a choice between making users unhappy and making developers
> > unhappy, I will side with the users each and every time.
>
> Well, let me rephrase. It is not only about me (as a developer) being
> unhappy but also all the side effects this would have for users when
> performance of their favorite workload declines for no apparent reason
> just because pcp caches are depleted by an unrelated process.
>
If depleted, we have a special worker that charge it. From the other hand,
the pcplist can be depleted by its nature, what _is_ not wrong. But just
in case we secure it since you had a concern about it.

Could you please specify a real test case or workload you are talking about?

Thank you for your comments and help.

--
Vlad Rezki