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

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Date:   Wed, 31 Aug 2022 12:47:32 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Kent Overstreet <kent.overstreet@...ux.dev>,
        Peter Zijlstra <peterz@...radead.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        akpm@...ux-foundation.org, 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,
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        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/30] Code tagging framework and applications

On Wed 31-08-22 11:19:48, Mel Gorman wrote:
> On Wed, Aug 31, 2022 at 04:42:30AM -0400, Kent Overstreet wrote:
> > On Wed, Aug 31, 2022 at 09:38:27AM +0200, Peter Zijlstra wrote:
> > > On Tue, Aug 30, 2022 at 02:48:49PM -0700, Suren Baghdasaryan wrote:
> > > > ===========================
> > > > Code tagging framework
> > > > ===========================
> > > > Code tag is a structure identifying a specific location in the source code
> > > > which is generated at compile time and can be embedded in an application-
> > > > specific structure. Several applications of code tagging are included in
> > > > this RFC, such as memory allocation tracking, dynamic fault injection,
> > > > latency tracking and improved error code reporting.
> > > > Basically, it takes the old trick of "define a special elf section for
> > > > objects of a given type so that we can iterate over them at runtime" and
> > > > creates a proper library for it.
> > > 
> > > I might be super dense this morning, but what!? I've skimmed through the
> > > set and I don't think I get it.
> > > 
> > > What does this provide that ftrace/kprobes don't already allow?
> > 
> > You're kidding, right?
> 
> It's a valid question. From the description, it main addition that would
> be hard to do with ftrace or probes is catching where an error code is
> returned. A secondary addition would be catching all historical state and
> not just state since the tracing started.
> 
> It's also unclear *who* would enable this. It looks like it would mostly
> have value during the development stage of an embedded platform to track
> kernel memory usage on a per-application basis in an environment where it
> may be difficult to setup tracing and tracking. Would it ever be enabled
> in production? Would a distribution ever enable this? If it's enabled, any
> overhead cannot be disabled/enabled at run or boot time so anyone enabling
> this would carry the cost without never necessarily consuming the data.
> 
> It might be an ease-of-use thing. Gathering the information from traces
> is tricky and would need combining multiple different elements and that
> is development effort but not impossible.
> 
> 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(-)

-- 
Michal Hocko
SUSE Labs