linux-kernel - Re: [RFC perf/core 05/11] uprobes: Add mapping for optimized uprobe trampolines

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Message-ID: <Zz-MI1J1FpvqItdq@J2N7QTR9R3>
Date: Thu, 21 Nov 2024 19:38:58 +0000
From: Mark Rutland <mark.rutland@....com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jiri Olsa <olsajiri@...il.com>, Oleg Nesterov <oleg@...hat.com>,
	Andrii Nakryiko <andrii@...nel.org>, bpf <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>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC perf/core 05/11] uprobes: Add mapping for optimized uprobe
 trampolines

[resending as I somehow messed up the 'From' header and got a tonne of
bounces]

On Thu, Nov 21, 2024 at 08:47:56AM -0800, Alexei Starovoitov wrote:
> On Thu, Nov 21, 2024 at 8:34 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > Elsewhere in the thread Mark Rutland already noted that arm64 really
> > doesn't need or want this.
> 
> Doesn't look like you've read what you quoted above.
> On arm64 the _HW_ cost may be the same.
> The _SW_ difference in handling trap vs syscall is real.
> I bet once uprobe syscall is benchmarked on arm64 there will
> be a delta.

I already pointed out in [1] that on arm64 we can make the trap case
*faster* than the syscall. If that's not already the case, there's only
a small amount of rework needed, (pulling BRK handling into
entry-common.c), which we want to do for other reasons anyway.

On arm64 I do not want the syscall; the trap is faster and simpler to
maintain.

Mark

[1] https://lore.kernel.org/lkml/ZzsRfhGSYXVK0mst@J2N7QTR9R3/