Re: [RFC PATCH 00/30] Code tagging framework and applications

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On Wed, Aug 31, 2022 at 12:02 PM Kent Overstreet
<kent.overstreet@xxxxxxxxx> wrote:
> On Wed, Aug 31, 2022 at 12:47:32PM +0200, Michal Hocko wrote:
> > On Wed 31-08-22 11:19:48, Mel Gorman wrote:
> > > 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(-)
> In the case of memory allocation accounting, you flat cannot do this with ftrace
> - you could maybe do a janky version that isn't fully accurate, much slower,
> more complicated for the developer to understand and debug and more complicated
> for the end user.
> But please, I invite anyone who's actually been doing this with ftrace to
> demonstrate otherwise.
> Ftrace just isn't the right tool for the job here - we're talking about adding
> per callsite accounting to some of the fastest fast paths in the kernel.
> And the size of the changes for memory allocation accounting are much more
> reasonable:
>  33 files changed, 623 insertions(+), 99 deletions(-)
> The code tagging library should exist anyways, it's been open coded half a dozen
> times in the kernel already.
> And once we've got that, the time stats code is _also_ far simpler than doing it
> with ftrace would be. If anyone here has successfully debugged latency issues
> with ftrace, I'd really like to hear it. Again, for debugging latency issues you
> want something that can always be on, and that's not cheap with ftrace - and
> never mind the hassle of correlating start and end wait trace events, builting
> up histograms, etc. - that's all handled here.
> Cheap, simple, easy to use. What more could you want?

This is very interesting work! Do you have any data about the overhead
this introduces, especially in a production environment? I am
especially interested in memory allocations tracking and detecting
(Sorry if you already posted this kind of data somewhere that I missed)

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