linux-kernel - Re: [RFC PATCH 00/30] Code tagging framework and applications

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Date:   Thu, 1 Sep 2022 09:36:58 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Mel Gorman <mgorman@...e.de>,
        Suren Baghdasaryan <surenb@...gle.com>,
        akpm@...ux-foundation.org, mhocko@...e.com, vbabka@...e.cz,
        hannes@...xchg.org, roman.gushchin@...ux.dev, dave@...olabs.net,
        willy@...radead.org, liam.howlett@...cle.com, void@...ifault.com,
        juri.lelli@...hat.com, ldufour@...ux.ibm.com, peterx@...hat.com,
        david@...hat.com, axboe@...nel.dk, mcgrof@...nel.org,
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        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/30] Code tagging framework and applications

On 9/1/22 09:05, Peter Zijlstra wrote:
>> Also, ftrace can drop events. Not really ideal if under system load your memory
>> accounting numbers start to drift.
> You could attach custom handlers to tracepoints. If you were to replace
> these unconditional code hooks of yours with tracepoints then you could
> conditionally (say at boot) register custom handlers that do the
> accounting you want.

That is strategy in RV (kernel/trace/rv/). It is in C, but I am also
adding support for monitors in bpf. The osnoise/timerlat tracers work this
way too, and they are enabled on Fedora/Red Hat/SUSE... production. They
will also be enabled in Ubuntu and Debian (the interwebs say).

The overhead of attaching code to tracepoints (or any "attachable thing") and
processing data in kernel is often lower than consuming it in user-space.
Obviously, when it is possible, e.g., when you respect locking rules, etc.

This paper (the basis for RV) shows a little comparison:
https://bristot.me/wp-content/uploads/2019/09/paper.pdf

By doing so, we also avoid problems of losing events... and you can also
generate other events from your attached code.

(It is also way easier to convince a maintainer to add a tracepoints or a trace
events than to add arbitrary code... ;-)

-- Daniel