Re: [PATCH] uprobes: reduce contention on uprobes_tree access

[Andrii Nakryiko]

On Mon, Mar 25, 2024 at 12:12 PM Jonthan Haslam
<jonathan.haslam@xxxxxxxxx> wrote:
>
> Hi Ingo,
>
> > > This change has been tested against production workloads that exhibit
> > > significant contention on the spinlock and an almost order of magnitude
> > > reduction for mean uprobe execution time is observed (28 -> 3.5 microsecs).
> >
> > Have you considered/measured per-CPU RW semaphores?
>
> No I hadn't but thanks hugely for suggesting it! In initial measurements
> it seems to be between 20-100% faster than the RW spinlocks! Apologies for
> all the exclamation marks but I'm very excited. I'll do some more testing
> tomorrow but so far it's looking very good.
>

Documentation ([0]) says that locking for writing calls
synchronize_rcu(), is that right? If that's true, attaching multiple
uprobes (including just attaching a single BPF multi-uprobe) will take
a really long time. We need to confirm we are not significantly
regressing this. And if we do, we need to take measures in the BPF
multi-uprobe attachment code path to make sure that a single
multi-uprobe attachment is still fast.

If my worries above turn out to be true, it still feels like a first
good step should be landing this patch as is (and get it backported to
older kernels), and then have percpu rw-semaphore as a final (and a
bit more invasive) solution (it's RCU-based, so feels like a good
primitive to settle on), making sure to not regress multi-uprobes
(we'll probably will need some batched API for multiple uprobes).

Thoughts?

  [0] https://docs.kernel.org/locking/percpu-rw-semaphore.html

> Thanks again for the input.
>
> Jon.