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

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Date:   Wed, 6 Jan 2021 11:27:12 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Phil Auld <pauld@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        vincent.donnefort@....com, Ingo Molnar <mingo@...hat.com>,
        vincent.guittot@...aro.org, LKML <linux-kernel@...r.kernel.org>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH v2] sched/debug: Add new tracepoint to track cpu_capacity

On 01/05/21 08:44, Alexei Starovoitov wrote:
> > Any pointer to an example test I could base this on?
> 
> selftests/bpf/

I was hoping for something more elaborate. I thought there's something already
there that do some verification for raw tracepoint that I could either extend
or replicate. Otherwise this could end up being a time sink for me and I'm not
keen on jumping down this rabbit hole.

> > > - add a doc with contents from commit log.
> >
> > You're referring to the ABI part of the changelog, right?
> >
> > > The "Does bpf make things into an abi ?" question keeps coming back
> > > over and over again.
> > > Everytime we have the same answer that No, bpf cannot bake things into abi.
> > > I think once it's spelled out somewhere in Documentation/ it would be easier to
> > > repeat this message.
> >
> > How about a new Documentation/bpf/ABI.rst? I can write something up initially
> > for us to discuss in detail when I post.
> 
> There is Documentation/bpf/bpf_design_QA.rst
> and we already have this text in there that was added back in 2017:
> 
> Q: Does BPF have a stable ABI?
> ------------------------------
> A: YES. BPF instructions, arguments to BPF programs, set of helper
> functions and their arguments, recognized return codes are all part
> of ABI. However there is one specific exception to tracing programs
> which are using helpers like bpf_probe_read() to walk kernel internal
> data structures and compile with kernel internal headers. Both of these
> kernel internals are subject to change and can break with newer kernels
> such that the program needs to be adapted accordingly.
> 
> I'm suggesting to add an additional section to this Q/A doc to include
> more or less
> the same text you had in the commit log.

Works for me.

Thanks

--
Qais Yousef