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

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Message-ID: <20240215193915.2d457718@gandalf.local.home>
Date: Thu, 15 Feb 2024 19:39:15 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Kent Overstreet <kent.overstreet@...ux.dev>
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Subject: Re: [PATCH v3 31/35] lib: add memory allocations report in
 show_mem()

On Thu, 15 Feb 2024 19:32:38 -0500
Kent Overstreet <kent.overstreet@...ux.dev> wrote:

> > 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?  
> 
> Microbenchmarks are how we magnify the effect of a change like this to
> the most we'll ever see. Barring cache effects, it'll be in the noise.
> 
> Cache effects are a concern here because we're now touching task_struct
> in the allocation fast path; that is where the
> "compiled-in-but-turned-off" overhead comes from, because we can't add
> static keys for that code without doubling the amount of icache
> footprint, and I don't think that would be a great tradeoff.
> 
> So: if your code has fastpath allocations where the hot part of
> task_struct isn't in cache, then this will be noticeable overhead to
> you, otherwise it won't be.

All nice, but where are the benchmarks? This looks like it will have an
affect on cache and you can talk all you want about how it will not be an
issue, but without real world benchmarks, it's meaningless. Numbers talk.

-- Steve