linux-kernel - Re: [PATCH 2/2] sched/debug: add sched_update_nr

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Date:   Thu, 5 Sep 2019 09:49:20 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Qais Yousef <qais.yousef@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jirka Hladký <jhladky@...hat.com>,
        Jiří Vozár <jvozar@...hat.com>,
        X86 ML <x86@...nel.org>
Subject: Re: [PATCH 2/2] sched/debug: add sched_update_nr_running tracepoint

On Thu, Sep 05, 2019 at 10:13:10AM +0200, Ingo Molnar wrote:
> 
> * Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
> > On Wed, Sep 4, 2019 at 10:47 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > On Wed, Sep 04, 2019 at 08:51:21AM -0700, Alexei Starovoitov wrote:
> > > > Anything in tracing can be deleted.
> > > > Tracing is about debugging and introspection.
> > > > When underlying kernel code changes the introspection points change as well.
> > >
> > > Right; except when it breaks widely used tools; like say powertop. Been
> > > there, done that.
> > 
> > powertop was a lesson learned, but it's not a relevant example anymore.
> > There are more widely used tools today. Like bcc tools.
> > And bpftrace is quickly gaining momentum and large user base.
> > bcc tools did break already several times and people fixed them.
> 
> Are these tools using libtraceevents?

bcc tools and bpftrace are using libbcc.
Which in turn is using libbpf.
libtraceevents is not used.

Interesting example is https://github.com/iovisor/bcc/blob/master/tools/tcplife.py
It's using "inet_sock_set_state" tracepoint when available on newer kernels
and kprobe in tcp_set_state() function on older kernels.
That tracepoint changed significantly over time.
It had different name 'tcp_set_state' and slightly different semantics.
Hence the tool was fixed when that change in tracepoint happened:
https://github.com/iovisor/bcc/commit/fd93dc0409b626b749b90f115d3d550a870ed125

Note that tcp:tcp_set_state tracepoint existed for full kernel release.
Yet people didn't make fuzz about the fact it disappeared in 4.16.
Though tcplife.py tool is simple there are more complex tools based
on this idea that are deployed in netflix and fb that went through the same
tcp_set_state->inet_sock_set_state fixes.