linux-kernel - Re: [RFC 00/11] uprobes: Add support to optimize usdt probes on x86

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Message-ID: <ZzsI5V4v33nyNqPG@krava>
Date: Mon, 18 Nov 2024 10:29:09 +0100
From: Jiri Olsa <olsajiri@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Oleg Nesterov <oleg@...hat.com>, Andrii Nakryiko <andrii@...nel.org>,
	bpf@...r.kernel.org, Song Liu <songliubraving@...com>,
	Yonghong Song <yhs@...com>,
	John Fastabend <john.fastabend@...il.com>,
	Hao Luo <haoluo@...gle.com>, Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Alan Maguire <alan.maguire@...cle.com>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>
Subject: Re: [RFC 00/11] uprobes: Add support to optimize usdt probes on
 x86_64

On Sun, Nov 17, 2024 at 12:49:46PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 05, 2024 at 02:33:54PM +0100, Jiri Olsa wrote:
> > hi,
> > this patchset adds support to optimize usdt probes on top of 5-byte
> > nop instruction.
> > 
> > The generic approach (optimize all uprobes) is hard due to emulating
> > possible multiple original instructions and its related issues. The
> > usdt case, which stores 5-byte nop seems much easier, so starting
> > with that.
> > 
> > The basic idea is to replace breakpoint exception with syscall which
> > is faster on x86_64. For more details please see changelog of patch 7.
> 
> So this is really about the fact that syscalls are faster than traps on
> x86_64? Is there something similar on ARM64, or are they roughly the
> same speed there?

yes, I recall somebody was porting uretprobe syscall to arm, but there was
no speed up IIRC, so looks like it's not the case on arm

I can't find the post atm, I'll keep digging

jirka

> 
> That is, I don't think this scheme will work for the various RISC
> architectures, given their very limited immediate range turns a typical
> call into a multi-instruction trainwreck real quick.
> 
> Now, that isn't a problem if their exceptions and syscalls are of equal
> speed.