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From:   Yafang Shao <laoar.shao@gmail.com>
Date:   Thu, 14 Oct 2021 23:05:17 +0800
Message-ID: <CALOAHbBaFYYvTDd4tFtYZznbNi-9pwXrNRrQb2iLa9st9M8wxw@mail.gmail.com>
Subject: Re: [sched.h] 317419b91e: perf-sanity-tests.Parse_sched_tracepoints_fields.fail
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        acme <acme@kernel.org>,
        kernel test robot <oliver.sang@intel.com>,
        0day robot <lkp@intel.com>, Petr Mladek <pmladek@suse.com>,
        Kees Cook <keescook@chromium.org>,
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        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        lkp <lkp@lists.01.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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On Thu, Oct 14, 2021 at 10:48 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Thu, 14 Oct 2021 22:40:04 +0800
> Yafang Shao <laoar.shao@gmail.com> wrote:
>
> > > mount -t tracefs nodev /sys/kernel/tracing
> > > cat /sys/kernel/tracing/events/sched/sched_switch/format
> > >
> > > name: sched_switch
> > > ID: 314
> > > format:
> > > [...]
> > >         field:char prev_comm[16];       offset:8;       size:16;        signed:1;
> > > [...]
> > >         field:char next_comm[16];       offset:40;      size:16;        signed:1;
> > >
> > > Both of those fields expose a fixed-size of 16 bytes.
> > >
> > > AFAIK Steven's intent was that by parsing this file, trace viewers could adapt to
> > > changes in the event field layout. Unfortunately, there have been cases where
> > > trace viewers had a hard expectation on the field layout. Hopefully those have
> > > all been fixed a while ago.
> > >
> >
> > I don't have a clear idea what will happen to trace viewers if we
> > extend task comm.
>
> There shouldn't be any doing a hard coded read of the events. That happened
> once with powertop, but they broke when they ran 32 bit userspace on a 64
> bit kernel, and they switched to libtraceevent to fix it. Which handles
> these updates.
>

Thanks for the explanation.

> >
> > Steven, do you have any suggestions ?
>
> The "Don't break user space" is a "tree in the forest" argument. We break
> user space all the time. But if no user space tool is around to hear it,
> did it really break? The answer is "no".
>

Got it!

-- 
Thanks
Yafang