linux-kernel - Re: [PATCH 00/10] perf/uprobe: Optimize uprobes

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Message-ID: <20240709153132.GR27299@noisy.programming.kicks-ass.net>
Date: Tue, 9 Jul 2024 17:31:32 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Masami Hiramatsu <mhiramat@...nel.org>, mingo@...nel.org,
	andrii@...nel.org, linux-kernel@...r.kernel.org,
	rostedt@...dmis.org, oleg@...hat.com, jolsa@...nel.org,
	clm@...a.com, bpf <bpf@...r.kernel.org>, willy@...radead.org
Subject: Re: [PATCH 00/10] perf/uprobe: Optimize uprobes

On Tue, Jul 09, 2024 at 07:36:41AM -0700, Paul E. McKenney wrote:

> > Per VMA refcounts or per VMA locks are a complete fail IMO.
> 
> Not even to allow concurrent updates of the address space by different
> threads of a process?

Well, I'm sure it helps some workloads. But for others it is just moving
the problem.

> For me, per-VMA locking's need to RCU-protect the VMA is a good step
> towards permitting RCU-protected scans of the Maple Tree, which then
> gets lockless lookup.

Right, the question is if the VMA lock is required to be stable against
splitting. If that is the case, we're hosed :/

At the time I added a seqcount for that, but I'm also remembering that's
one of the things people complained about for single threaded
performance.