linux-kernel - Re: [PATCH v1] dynamic_debug: add support for logs destination

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Message-ID: <CAJfuBxyRF4q_T8LmHwR=-PKKDDpiFg2nO03uLnL8aGpRyBByKw@mail.gmail.com>
Date:   Fri, 6 Oct 2023 14:49:27 -0600
From:   jim.cromie@...il.com
To:     Łukasz Bartosik <lb@...ihalf.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Jason Baron <jbaron@...mai.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Douglas Anderson <dianders@...omium.org>,
        Guenter Roeck <groeck@...gle.com>,
        Yaniv Tzoreff <yanivt@...gle.com>,
        Benson Leung <bleung@...gle.com>, linux-kernel@...r.kernel.org,
        upstream@...ihalf.com,
        Vincent Whitchurch <vincent.whitchurch@...s.com>
Subject: Re: [PATCH v1] dynamic_debug: add support for logs destination

On Wed, Oct 4, 2023 at 4:55 AM Łukasz Bartosik <lb@...ihalf.com> wrote:
>
> wt., 3 paź 2023 o 22:54 <jim.cromie@...il.com> napisał(a):
> >
> > On Tue, Oct 3, 2023 at 1:57 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> > >
> > > On Mon, 2 Oct 2023 14:49:20 -0600
> > > jim.cromie@...il.com wrote:
> > >
> > > > hi Lukasz,
> > > >
> > > > sorry my kernel-time has been in my own trees.
> > > >
> > > > What I dont understand is why +T is insufficient.
> > > >
>
> We would like to be able to separate debug logs from different
> subsystem (e.g. thunderbolt and usbcore).
> With +T it is not possible because all debug logs will land in the same bucket.
>
> > > > IIUC, tracefs is intended for production use.
> > > > thats why each event can be enabled / disabled
> > > > - to select and minimize whats traced, and not impact the system
> > > >
> > > > and +T  can forward all pr_debugs to trace,
> > > > (by 1-few trace events defined similarly to others)
> > > > or very few, giving yet another selection mechanism
> > > > to choose or eliminate specific pr-debugs and reduce traffic to
> > > > interesting stuff.
> > > >
> > > > Once your debug is in the trace-buf,
> > > > shouldnt user-space be deciding what to do with it ?
> > > > a smart daemon could leverage tracefs to good effect.
> > > >
>
> Yes, a daemon could separate the debug logs but IMHO it is much
> easier to separate logs by sending them directly from a given subsystem
> to a separate trace instance. My proposal allows to configure different
> trace instance as destination for each callsite.
>
> > > > IMO the main value of +T is that it allows feeding existing pr_debugs
> > > > into the place where other trace-data is already integrated and managed.
> > > >
> > > > At this point, I dont see any extra destination handling as prudent.
> > > >
> > >
> > >
> > > I'm fine with either approach. I kind of like the creation of the instance,
> > > as that allows the user to keep this debug separate from other tracing
> > > going on. We are starting to have multiple applications using the tracing
> > > buffer (although most are using instances, which is why I'm trying to make
> > > them lighter weight with the eventfs code).
> > >
> > > -- Steve
> > >
>
> Steve, thanks for commenting from the trace perspective.
>
> >
> >
> > Ok Im starting to grasp that multiple instances are good
> > (and wondering how I didnt notice)
> >
> > What doesnt thrill me is the new _ddebug field, it enlarges the footprint.
> >
>
> Yes it increases _ddebug structure by a pointer size.
>
> > can you make it go away ?
>
> I implemented my proposal with flexibility in mind so that if someone
> would like to add
> another destination in the future it should be easy to do. I
> understand that adding a pointer
> to the _ddebug structure increases footprint size that's why I also
> added CONFIG_DYNAMIC_DEBUG_DST
> kernel configuration option in order to enable/disable this functionality.
>
> > I have some thoughts ..
>
> Please share your thoughts. I'm sure we can come to an agreement how
> to incorporate both +T and my proposal.


So heres what Im thinking:

shrink lineno, get 2-3 bits back.
last I checked largest C file is <32kloc
largest header is ~120kloc, but its a data only,
no pr_debugs will suddenly appear there.

define a dst_id field with 3 bits
0 is for main tracebuf
1-7 is for other instances

then the alt-dest lookup is avoided except when the dst_id field is >0

It might work to put the alt-dst-pointer into the classmaps,
so the destination is used for the entire group of debugs
forex DRM_UT_CORE etc.

But its no better than the dst_id field, which is per-callsite,
and entirely independent of classes.


> Thanks,
> Lukasz