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

Uladzislau Rezki
2020-09-22T13:13:05Z

> > > > 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.
>
> Yes, this is not trivial but not a rocket science either. Remember that
> you are relying on a very dumb watermark based pcp pool from the
> allocator.
>
We rely on it, indeed. If the pcp-cache is depleted our special work is
triggered to charge our local cache(few pages) such way will also initiate
the process of pre-featching pages from the buddy allocator populating
the depleted pcp-cache. I do not have any concern here.

>
> Mimicing a similar implementation shouldn't be all that hard
> and you will get your own pool which doesn't affect other page allocator
> users as much as a bonus.
>
I see your point Michal. As i mentioned before, it is important to avoid of
having such own pools, because the aim is not to waste memory resources. A
page will be returned back to "page allocator" as soon as a scheduler place
our reclaim thread on a CPU and grace period is passed. So, the resource
can be used for other needs. What is important.

Otherwise a memory footprint is increased what is bad for low memory
conditions when OOM is involved. Just in case, it is a big issue for
mobile devices.

> > b) Memory overhead since we do not know how much pages should be
> > preloaded: 100, 200 or 300
>
> Does anybody who really needs this optimization actually cares about 300
> pages?
>
It might be an issue for embedded devices when such devices run into a
low memory condition resulting in OOM or slow allocations due to mentioned
condition. For servers and big system it will not be visible.

> > 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.
>
> How big "machines" are we talking about here? I would expect that really
> tiny machines would have hard times to really fill up thousands of pages
> with pointers to free...
>
I mentioned above. We can not rely on static model. We would like to
have a mechanism that gives back ASAP used pages to page allocator
for other needs.

>
> Would a similar scaling as the page allocator feasible. Really I mostly
> do care about shared nature of the pcp allocator list that one user can
> easily monopolize with this API.
>
I see your concern. pcplist can be monopolized by already existing API:

while (i < 100)
__get_free_page(GFP_NOWAIT | __GFP_NOWARN);

> > > 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.
>
> This should be part of the changelog.
>
Hmm.. I think it is. Sorry if i missed that but i hope i mentioned about it.

> >
> > Single-argument details is here: https://lkml.org/lkml/2020/4/28/1626
>
> Error 501
>
Could you please elaborate? Do not want to speculate :)

> > > > > 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.
>
> pcp free lists should ever get empty when we run out of memory and need
> to reclaim. Otherwise they are constantly refilled/rebalanced on demand.
> The fact that you are refilling them from outside just suggest that you
> are operating on a wrong layer. Really, create your own pool of pages
> and rebalance them based on the workload.
>
I covered it above.

> > Could you please specify a real test case or workload you are talking about?
>
> I am not a performance expert but essentially any memory allocator heavy
> workload might notice. I am pretty sure Mel would tell you more.
>
OK.

Thank you for your comments, Michal!

--
Vlad Rezki