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

Roman Gushchin
2022-09-01T22:30:05Z

On Wed, Aug 31, 2022 at 01:56:08PM -0700, Yosry Ahmed wrote:
> On Wed, Aug 31, 2022 at 12:02 PM Kent Overstreet
> <kent.overstreet@linux.dev> 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
> leaks.

+1

I think the question whether it indeed can be always turned on in the production
or not is the main one. If not, the advantage over ftrace/bpf/... is not that
obvious. Otherwise it will be indeed a VERY useful thing.

Also, there is a lot of interesting stuff within this patchset, which
might be useful elsewhere. So thanks to Kent and Suren for this work!

Thanks!