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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Wed, 31 Aug 2022 08:28:40 -0700
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Subject: Re: [RFC PATCH 00/30] Code tagging framework and applications
To:     Michal Hocko <mhocko@suse.com>
Cc:     Mel Gorman <mgorman@suse.de>,
        Kent Overstreet <kent.overstreet@linux.dev>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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On Wed, Aug 31, 2022 at 3:47 AM Michal Hocko <mhocko@suse.com> wrote:
>
> 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.

Thank you for the question.
For memory tracking my intent is to have a mechanism that can be enabled in
the field testing (pre-production testing on a large population of
internal users).
The issue that we are often facing is when some memory leaks are happening
in the field but very hard to reproduce locally. We get a bugreport
from the user
which indicates it but often has not enough information to track it. Note that
quite often these leaks/issues happen in the drivers, so even simply finding out
where they came from is a big help.
The way I envision this mechanism to be used is to enable the basic memory
tracking in the field tests and have a user space process collecting
the allocation
statistics periodically (say once an hour). Once it detects some counter growing
infinitely or atypically (the definition of this is left to the user
space) it can enable
context capturing only for that specific location, still keeping the
overhead to the
minimum but getting more information about potential issues. Collected stats and
contexts are then attached to the bugreport and we get more visibility
into the issue
when we receive it.
The goal is to provide a mechanism with low enough overhead that it
can be enabled
all the time during these field tests without affecting the device's
performance profiles.
Tracing is very cheap when it's disabled but having it enabled all the
time would
introduce higher overhead than the counter manipulations.
My apologies, I should have clarified all this in this cover letter
from the beginning.

As for other applications, maybe I'm not such an advanced user of
tracing but I think only
the latency tracking application might be done with tracing, assuming
we have all the
right tracepoints but I don't see how we would use tracing for fault
injections and
descriptive error codes. Again, I might be mistaken.

Thanks,
Suren.

> >
> > 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