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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date: Mon, 25 Mar 2024 16:14:18 -0700
Message-ID: <CAEf4BzZnUcFEk+7r39n8=zopBeK96wfQ4Vtndd2tY2ymqOpCtA@mail.gmail.com>
Subject: Re: [PATCH] uprobes: reduce contention on uprobes_tree access
To: Jonthan Haslam <jonathan.haslam@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>, linux-trace-kernel@vger.kernel.org, andrii@kernel.org, 
	bpf@vger.kernel.org, rostedt@goodmis.org, mhiramat@kernel.org, 
	Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>, 
	Arnaldo Carvalho de Melo <acme@kernel.org>, Namhyung Kim <namhyung@kernel.org>, 
	Mark Rutland <mark.rutland@arm.com>, 
	Alexander Shishkin <alexander.shishkin@linux.intel.com>, Jiri Olsa <jolsa@kernel.org>, 
	Ian Rogers <irogers@google.com>, Adrian Hunter <adrian.hunter@intel.com>, 
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org
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On Mon, Mar 25, 2024 at 12:12=E2=80=AFPM Jonthan Haslam
<jonathan.haslam@gmail.com> wrote:
>
> Hi Ingo,
>
> > > This change has been tested against production workloads that exhibit
> > > significant contention on the spinlock and an almost order of magnitu=
de
> > > reduction for mean uprobe execution time is observed (28 -> 3.5 micro=
secs).
> >
> > 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 fo=
r
> 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.