Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:6 as permitted sender) client-ip=2620:137:e000::3:6;
MIME-Version: 1.0
References: <20230915154856.1896062-1-lb@semihalf.com> <CAJfuBxyFYyGCtr5i=P7N=1oX3J=jmdp1VLGLt+z1fAnuvGK2aA@mail.gmail.com>
 <CAK8ByeJBrPEQSgUc91LQO9Krzjh2pauhMTjEC82M8ozayE76Yg@mail.gmail.com>
 <CAJfuBxxmL-GtBgt=033F9UNeLCreFbJh3HrQQN2nYKwR_0uTbg@mail.gmail.com>
 <20231003155810.6df9de16@gandalf.local.home> <CAJfuBxyJyFbFEhRxrtxJ_RazaTODV6Gg64b1aiNEzt6_iE4=Og@mail.gmail.com>
In-Reply-To: <CAJfuBxyJyFbFEhRxrtxJ_RazaTODV6Gg64b1aiNEzt6_iE4=Og@mail.gmail.com>
From:   =?UTF-8?Q?=C5=81ukasz_Bartosik?= <lb@semihalf.com>
Date:   Wed, 4 Oct 2023 12:55:01 +0200
Message-ID: <CAK8ByeLNc9UbTNG4x=40AxYqjjRCsvBNtNFai0PMveM2X4XCow@mail.gmail.com>
Subject: Re: [PATCH v1] dynamic_debug: add support for logs destination
To:     jim.cromie@gmail.com
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Jason Baron <jbaron@akamai.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Douglas Anderson <dianders@chromium.org>,
        Guenter Roeck <groeck@google.com>,
        Yaniv Tzoreff <yanivt@google.com>,
        Benson Leung <bleung@google.com>, linux-kernel@vger.kernel.org,
        upstream@semihalf.com,
        Vincent Whitchurch <vincent.whitchurch@axis.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

wt., 3 pa=C5=BA 2023 o 22:54 <jim.cromie@gmail.com> napisa=C5=82(a):
>
> On Tue, Oct 3, 2023 at 1:57=E2=80=AFPM Steven Rostedt <rostedt@goodmis.or=
g> wrote:
> >
> > On Mon, 2 Oct 2023 14:49:20 -0600
> > jim.cromie@gmail.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 buc=
ket.

> > > 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 manag=
ed.
> > >
> > > 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 insta=
nce,
> > as that allows the user to keep this debug separate from other tracing
> > going on. We are starting to have multiple applications using the traci=
ng
> > buffer (although most are using instances, which is why I'm trying to m=
ake
> > 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.

Thanks,
Lukasz