Re: [PATCH 2/2] trace-cmd: Add an option to set saved_cmdlines_size

[Vincent Donnefort <vincent.donnefort@arm.com>]

On Tue, Nov 12, 2019 at 09:40:37AM -0500, Steven Rostedt wrote:
> On Tue, 12 Nov 2019 12:03:15 +0000
> Vincent Donnefort <vincent.donnefort@arm.com> wrote:
>
> > Hi Steve,
> >
> > Thank you for your review. I also thought that we could avoid having a new
> > option and try to write /proc/sys/kernel/threads-max into saved_cmdlines_size,
> > before recording the trace. Do you think that would be a more suitable
> > solution?
>
> That's an interesting approach. I still think that having a parameter
> would be good as well, but then use the threads-max approach if one is
> not supplied.
>
> -- Steve

This was not a good idea in the end. We can't rely on the sysctl entries
threads-max or pid_max, as the pid/comm map is static and the maximum is
defined by PID_MAX_DEFAULT. I do not see any way of getting this value from
user-space.

We could though, as an alternative, use a hard-coded value of 32768 for
--cmdlines-size. Without being perfect, this should still be true for most of
the systems. PID_MAX_DEFAULT depends on CONFIG_BASE_SMALL and hasn't changed
since Linux started using Git.

--
Vincent
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