linux-kernel - Re: [PATCH v4 1/1] perf/kprobe: maxactive for fd-based kprobe

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Date:   Thu, 14 Jul 2022 21:57:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitrii Dolgov <9erthalion6@...il.com>
Cc:     linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        songliubraving@...com, rostedt@...dmis.org, mingo@...hat.com,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v4 1/1] perf/kprobe: maxactive for fd-based kprobe

On Thu, Jul 14, 2022 at 09:34:03PM +0200, Dmitrii Dolgov wrote:
> From: Song Liu <songliubraving@...com>
> 
> Enable specifying maxactive for fd based kretprobe. This will be useful
> for tracing tools like bcc and bpftrace (see for example discussion [1]).
> Use highest 4 bit (bit 59-63) to allow specifying maxactive by log2.

What's maxactive? This doesn't really tell me much. Why are the top 4
bits the best to use?

> 
> The original patch [2] seems to be fallen through the cracks and wasn't
> applied. I've merely rebased the work done by Song Liu, verififed it
> still works, and modified to allow specifying maxactive by log2 per
> suggestion from the discussion thread.

That just doesn't belong in a Changelog.

> 
> Note that changes in rethook implementation may render maxactive
> obsolete.

Then why bother creating an ABI for it?

> 
> [1]: https://github.com/iovisor/bpftrace/issues/835
> [2]: https://lore.kernel.org/all/20191007223111.1142454-1-songliubraving@fb.com/
> 
> Signed-off-by: Song Liu <songliubraving@...com>
> Signed-off-by: Dmitrii Dolgov <9erthalion6@...il.com>
> Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> Acked-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Lots of question and not a single answer in sight...