Re: [PATCH 00/40] Memory allocation profiling

From: "Petr Tesařík" <petr@tesarici.cz>
To: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Michal Hocko <mhocko@suse.com>,
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Subject: Re: [PATCH 00/40] Memory allocation profiling
Date: Mon, 8 May 2023 18:09:13 +0200	[thread overview]
Message-ID: <20230508180913.6a018b21@meshulam.tesarici.cz> (raw)
In-Reply-To: <ZFkb1p80vq19rieI@moria.home.lan>

On Mon, 8 May 2023 11:57:10 -0400
Kent Overstreet <kent.overstreet@linux.dev> wrote:

> On Mon, May 08, 2023 at 05:52:06PM +0200, Petr Tesařík wrote:
> > On Sun, 7 May 2023 13:20:55 -0400
> > Kent Overstreet <kent.overstreet@linux.dev> wrote:
> >   
> > > 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  
> > 
> > Thank you. That's exactly what I was trying to point out. So you hook
> > seq_buf_alloc(), just to find out it's called from traverse(), which
> > is not very helpful either. So, you hook traverse(), which sounds quite
> > generic. Yes, you're lucky, because it is a static function, and the
> > identifier is not actually used anywhere else (right now), but each
> > time you want to hook something, you must make sure it does not
> > conflict with any other identifier in the kernel...  
> 
> Cscope makes quick and easy work of this kind of stuff.

Sure, although AFAIK the index does not cover all possible config
options (so non-x86 arch code is often forgotten). However, that's the
less important part.

What do you do if you need to hook something that does conflict with an
existing identifier?

Petr T