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

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Message-ID: <20240709142943.GL27299@noisy.programming.kicks-ass.net>
Date: Tue, 9 Jul 2024 16:29:43 +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 07:11:23AM -0700, Paul E. McKenney wrote:
> On Tue, Jul 09, 2024 at 11:01:53AM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 08, 2024 at 05:25:14PM -0700, Andrii Nakryiko wrote:
> > 
> > > Quick profiling for the 8-threaded benchmark shows that we spend >20%
> > > in mmap_read_lock/mmap_read_unlock in find_active_uprobe. I think
> > > that's what would prevent uprobes from scaling linearly. If you have
> > > some good ideas on how to get rid of that, I think it would be
> > > extremely beneficial. 
> > 
> > That's find_vma() and friends. I started RCU-ifying that a *long* time
> > ago when I started the speculative page fault patches. I sorta lost
> > track of that effort, Willy where are we with that?
> > 
> > Specifically, how feasible would it be to get a simple RCU based
> > find_vma() version sorted these days?
> 
> Liam's and Willy's Maple Tree work, combined with Suren's per-VMA locking
> combined with some of Vlastimil's slab work is pushing in that direction.
> I believe that things are getting pretty close.

So I fundamentally do not believe in per-VMA locking. Specifically for
this case that would be trading one hot line for another. I tried
telling people that, but it doesn't seem to stick :/

Per VMA refcounts or per VMA locks are a complete fail IMO.

I suppose I should go dig out the latest versions of those patches to
see where they're at :/