linux-kernel - Re: [PATCH v3 31/35] lib: add memory allocations report in show

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Date: Thu, 15 Feb 2024 19:21:41 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Vlastimil Babka <vbabka@...e.cz>, Suren Baghdasaryan
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Subject: Re: [PATCH v3 31/35] lib: add memory allocations report in
 show_mem()

On Thu, 15 Feb 2024 18:51:41 -0500
Kent Overstreet <kent.overstreet@...ux.dev> wrote:

> Most of that is data (505024), not text (68582, or 66k).
> 

And the 4K extra would have been data too.

> The data is mostly the alloc tags themselves (one per allocation
> callsite, and you compiled the entire kernel), so that's expected.
> 
> Of the text, a lot of that is going to be slowpath stuff - module load
> and unload hooks, formatt and printing the output, other assorted bits.
> 
> Then there's Allocation and deallocating obj extensions vectors - not
> slowpath but not super fast path, not every allocation.
> 
> The fastpath instruction count overhead is pretty small
>  - actually doing the accounting - the core of slub.c, page_alloc.c,
>    percpu.c
>  - setting/restoring the alloc tag: this is overhead we add to every
>    allocation callsite, so it's the most relevant - but it's just a few
>    instructions.
> 
> So that's the breakdown. Definitely not zero overhead, but that fixed
> memory overhead (and additionally, the percpu counters) is the price we
> pay for very low runtime CPU overhead.

But where are the benchmarks that are not micro-benchmarks. How much
overhead does this cause to those? Is it in the noise, or is it noticeable?

-- Steve