linux-kernel - Re: [PATCH v2] sched/debug: Add new tracepoint to track cpu

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Message-ID: <20200907104845.6rust2lf2o3d5gmq@e107158-lin.cambridge.arm.com>
Date:   Mon, 7 Sep 2020 11:48:45 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     vincent.donnefort@....com, mingo@...hat.com, peterz@...radead.org,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        valentin.schneider@....com, Phil Auld <pauld@...hat.com>
Subject: Re: [PATCH v2] sched/debug: Add new tracepoint to track cpu_capacity

Hi Dietmar

On 09/02/20 12:44, Dietmar Eggemann wrote:
> + Phil Auld <pauld@...hat.com>
> 
> On 28/08/2020 19:26, Qais Yousef wrote:
> > On 08/28/20 19:10, Dietmar Eggemann wrote:
> >> On 28/08/2020 12:27, Qais Yousef wrote:
> >>> On 08/28/20 10:00, vincent.donnefort@....com wrote:
> >>>> From: Vincent Donnefort <vincent.donnefort@....com>
> 
> [...]
> 
> >> Can you remind me why we have all these helper functions like
> >> sched_trace_rq_cpu_capacity?
> > 
> > struct rq is defined in kernel/sched/sched.h. It's not exported. Exporting
> > these helper functions was the agreement to help modules trace internal info.
> > By passing generic info you decouple the tracepoint from giving specific info
> > and allow the modules to extract all the info they need from the same
> > tracepoint. IE: if you need more than just cpu_capacity from this tracepoint,
> > you can get that without having to continuously add extra arguments everytime
> > you need an extra piece of info. Unless this info is not in the rq of course.
> 
> I think this decoupling is not necessary. The natural place for those
> scheduler trace_event based on trace_points extension files is
> kernel/sched/ and here the internal sched.h can just be included.
> 
> If someone really wants to build this as an out-of-tree module there is
> an easy way to make kernel/sched/sched.h visible.
> 
> CFLAGS_sched_tp.o := -I$KERNEL_SRC/kernel/sched
> 
> all:
>     make -C $KERNEL_SRC M=$(PWD) modules

Sorry for the late response. Was on holiday.

IMHO the above is a hack. Out-of-tree modules should rely on public headers and
exported functions only. What you propose means that people who want to use
these tracepoints in meaningful way must have a prebuilt kernel handy. Which is
maybe true for us who work in the embedded world. But users who run normal
distro kernels (desktop/servers) will fail to build against
`/lib/modules/$(uname -r)/build` where that internal header file is not
exported. IOW, we're putting extra hoops for a large class of users here to be
able to access these internal data. They have to maintain their out-of-tree
definition of these structures.

FWIW, I did raise this concern with Peter in 2019 OSPM and he was okay with the
exports as it's still not a contract and they can disappear anytime we want.
Migrating to using BTF is the right way forward IMO. I don't think what we have
here is out-of-control yet. Though I agree they're annoying.

Cheers

--
Qais Yousef