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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date: Sun, 24 Mar 2024 04:28:54 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Jonathan Haslam <jonathan.haslam@gmail.com>
Cc: 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>,
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	Adrian Hunter <adrian.hunter@intel.com>,
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] uprobes: reduce contention on uprobes_tree access
Message-ID: <Zf+d9twfyIDosINf@gmail.com>
References: <20240321145736.2373846-1-jonathan.haslam@gmail.com>
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* Jonathan Haslam <jonathan.haslam@gmail.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