Message-ID: <ZFfd99w9vFTftB8D@moria.home.lan>
Date:   Sun, 7 May 2023 13:20:55 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Michal Hocko <mhocko@...e.com>
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Subject: Re: [PATCH 00/40] Memory allocation profiling

On Thu, May 04, 2023 at 11:07:22AM +0200, Michal Hocko wrote:
> No. I am mostly concerned about the _maintenance_ overhead. For the
> bare tracking (without profiling and thus stack traces) only those
> allocations that are directly inlined into the consumer are really
> of any use. That increases the code impact of the tracing because any
> relevant allocation location has to go through the micro surgery. 
> 
> e.g. is it really interesting to know that there is a likely memory
> leak in seq_file proper doing and allocation? No as it is the specific
> implementation using seq_file that is leaking most likely. There are
> other examples like that See?

So this is a rather strange usage of "maintenance overhead" :)

But it's something we thought of. If we had to plumb around a _RET_IP_
parameter, or a codetag pointer, it would be a hassle annotating the
correct callsite.

Instead, alloc_hooks() wraps a memory allocation function and stashes a
pointer to a codetag in task_struct for use by the core slub/buddy
allocator code.

That means that in your example, to move tracking to a given seq_file
function, we just:
 - hook the seq_file function with alloc_hooks
 - change the seq_file function to call non-hooked memory allocation
   functions.

> It would have been more convincing if you had some numbers at hands.
> E.g. this is a typical workload we are dealing with. With the compile
> time tags we are able to learn this with that much of cost. With a dynamic
> tracing we are able to learn this much with that cost. See? As small as
> possible is a rather vague term that different people will have a very
> different idea about.

Engineers don't prototype and benchmark everything as a matter of
course, we're expected to have the rough equivealent of a CS education
and an understanding of big O notation, cache architecture, etc.

The slub fast path is _really_ fast - double word non locked cmpxchg.
That's what we're trying to compete with. Adding a big globally
accessible hash table is going to tank performance compared to that.

I believe the numbers we already posted speak for themselves. We're
considerably faster than memcg, fast enough to run in production.

I'm not going to be switching to a design that significantly regresses
performance, sorry :)

> TBH I am much more concerned about the maintenance burden on the MM side
> than the actual code tagging itslef which is much more self contained. I
> haven't seen other potential applications of the same infrastructure and
> maybe the code impact would be much smaller than in the MM proper. Our
> allocator API is really hairy and convoluted.

You keep saying "maintenance burden", but this is a criticism that can
be directed at _any_ patchset that adds new code; it's generally
understood that that is the accepted cost for new functionality.

If you have specific concerns where you think we did something that
makes the code harder to maintain, _please point them out in the
appropriate patch_. I don't think you'll find too much - the
instrumentation in the allocators simply generalizes what memcg was
already doing, and the hooks themselves are a bit boilerplaty but hardly
the sort of thing people will be tripping over later.

TL;DR - put up or shut up :)

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