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

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Message-ID: <CAJD7tkaev9B=UDYj2RL6pz-1454J8tv4gEr9y-2dnCksoLK0bw@mail.gmail.com>
Date:   Wed, 31 Aug 2022 13:56:08 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Michal Hocko <mhocko@...e.com>, Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>, dave@...olabs.net,
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        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 00/30] Code tagging framework and applications

On Wed, Aug 31, 2022 at 12:02 PM Kent Overstreet
<kent.overstreet@...ux.dev> wrote:
>
> On Wed, Aug 31, 2022 at 12:47:32PM +0200, Michal Hocko wrote:
> > On Wed 31-08-22 11:19:48, Mel Gorman wrote:
> > > Whatever asking for an explanation as to why equivalent functionality
> > > cannot not be created from ftrace/kprobe/eBPF/whatever is reasonable.
> >
> > Fully agreed and this is especially true for a change this size
> > 77 files changed, 3406 insertions(+), 703 deletions(-)
>
> In the case of memory allocation accounting, you flat cannot do this with ftrace
> - you could maybe do a janky version that isn't fully accurate, much slower,
> more complicated for the developer to understand and debug and more complicated
> for the end user.
>
> But please, I invite anyone who's actually been doing this with ftrace to
> demonstrate otherwise.
>
> Ftrace just isn't the right tool for the job here - we're talking about adding
> per callsite accounting to some of the fastest fast paths in the kernel.
>
> And the size of the changes for memory allocation accounting are much more
> reasonable:
>  33 files changed, 623 insertions(+), 99 deletions(-)
>
> The code tagging library should exist anyways, it's been open coded half a dozen
> times in the kernel already.
>
> And once we've got that, the time stats code is _also_ far simpler than doing it
> with ftrace would be. If anyone here has successfully debugged latency issues
> with ftrace, I'd really like to hear it. Again, for debugging latency issues you
> want something that can always be on, and that's not cheap with ftrace - and
> never mind the hassle of correlating start and end wait trace events, builting
> up histograms, etc. - that's all handled here.
>
> Cheap, simple, easy to use. What more could you want?
>

This is very interesting work! Do you have any data about the overhead
this introduces, especially in a production environment? I am
especially interested in memory allocations tracking and detecting
leaks.
(Sorry if you already posted this kind of data somewhere that I missed)