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

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Message-ID: <7505621f-23ce-4b50-9e0d-84c9e7754444@paulmck-laptop>
Date: Tue, 9 Jul 2024 08:56:47 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Peter Zijlstra <peterz@...radead.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 05:31:32PM +0200, Peter Zijlstra wrote:
> 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.

>From where I sit, helping a wide range of workloads is a good thing.  ;-)

> > 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 :/

Let's just say that VMA splitting and merging has consumed much time
and effort from the usual suspects over the past while.

> 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.

Sequence locks are lighter weight these days, but again, this has been
very much a long-term whack-a-mole exercise with odd regressions.

							Thanx, Paul