[RFC 0/9] Nginx refcount scalability issue with Apparmor enabled and potential solutions

[Mateusz Guzik]

On Wed, May 29, 2024 at 2:37 AM John Johansen
<john.johansen at canonical.com> wrote:
> I don't have objections to moving towards percpu refcounts, but the overhead
> of a percpu stuct per label is a problem when we have thousands of labels
> on the system. That is to say, this would have to be a config option. We
> moved buffers from kmalloc to percpu to reduce memory overhead to reduce
> contention. The to percpu, to a global pool because the percpu overhead was
> too high for some machines, and then from a global pool to a hybrid scheme
> because of global lock contention. I don't see a way of doing that with the
> label, which means a config would be the next best thing.
>

There was a patchset somewhere which adds counters starting as atomic
and automagically converting themselves per-cpu if there as enough
load applied to them. General point being it is plausible this may
autotune itself.

Another option would be a boot-time tunable.

> Not part of your patch but something to be considered is that the label tree
> needs a rework, its locking needs to move to read side a read side lock less
> scheme, and the plan was to make it also use a linked list such that new
> labels are always queued at the end, allowing dynamically created labels to
> be lazily added to the tree.
>

It's not *my* patchset. ;)

> I see the use of the kworker as problematic as well, especially if we are
> talking using kconfig to switch reference counting modes. I am futzing with
> some ideas, on how to deal with this.
>

Thanks for the update. Hopefully this is going to get sorted out in
the foreseeable future.

-- 
Mateusz Guzik <mjguzik gmail.com>