linux-kernel - Re: [PATCH] uprobes: reduce contention on uprobes

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Date: Sun, 24 Mar 2024 04:28:54 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Jonathan Haslam <jonathan.haslam@...il.com>
Cc: linux-trace-kernel@...r.kernel.org, andrii@...nel.org,
	bpf@...r.kernel.org, rostedt@...dmis.org, mhiramat@...nel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] uprobes: reduce contention on uprobes_tree access


* Jonathan Haslam <jonathan.haslam@...il.com> wrote:

> Active uprobes are stored in an RB tree and accesses to this tree are
> dominated by read operations. Currently these accesses are serialized by
> a spinlock but this leads to enormous contention when large numbers of
> threads are executing active probes.
> 
> This patch converts the spinlock used to serialize access to the
> uprobes_tree RB tree into a reader-writer spinlock. This lock type
> aligns naturally with the overwhelmingly read-only nature of the tree
> usage here. Although the addition of reader-writer spinlocks are
> discouraged [0], this fix is proposed as an interim solution while an
> RCU based approach is implemented (that work is in a nascent form). This
> fix also has the benefit of being trivial, self contained and therefore
> simple to backport.
> 
> 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?

Thanks,

	Ingo