On Thu, 2013-04-25 at 18:51 +0900, Namhyung Kim wrote:
> >
> >>
> >> Do you mean it should be changed to
> >>
> >> if (write_tracing_file("set_ftrace_pid", "-1") < 0)
> >>
> >> ? It seems it's the right thing to do for compatibility.
> >
> >
> > looks like it's enough only to open it with O_TRUNC,
> > so I guess whatever minimal goes write_tracing_file
Correct.
>
> Yeah, but I can see below from trace-cmd code.
>
> trace-cmd/trace-record.c:
>
> /*
> * Older kernels required "-1" to disable pid
> */
> if (ret < 0 && !strlen(pid))
> ret = write(fd, "-1", 2);
>
>
> Could you clarify this, Steve?
As the comment states, older kernels (before 2.6.33) required a -1. As
trace-cmd is set up to work with at least 2.6.32, it requires the "-1"
write. It doesn't hurt to have it.
But for perf ftrace (or whatever), I don't think we need the requirement
to work with 2.6.32 kernels. Perf was added in 2.6.31, but do all
functionalities still work that far back?
-- Steve
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