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

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Date:   Thu, 1 Sep 2022 15:27:27 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Kent Overstreet <kent.overstreet@...ux.dev>,
        Michal Hocko <mhocko@...e.com>, Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>, dave@...olabs.net,
        Matthew Wilcox <willy@...radead.org>, liam.howlett@...cle.com,
        void@...ifault.com, juri.lelli@...hat.com, ldufour@...ux.ibm.com,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>, axboe@...nel.dk,
        mcgrof@...nel.org, masahiroy@...nel.org, nathan@...nel.org,
        changbin.du@...el.com, ytcoode@...il.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
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        bristot@...hat.com, vschneid@...hat.com,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
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        Muchun Song <songmuchun@...edance.com>, arnd@...db.de,
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        kernel-team@...roid.com, Linux-MM <linux-mm@...ck.org>,
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        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 00/30] Code tagging framework and applications

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@...ux.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!