Re: Another Obsolete Fix me in trace.h?

Steven Rostedt
2014-11-24T21:19:05Z

On Mon, 24 Nov 2014 22:00:01 +0100
Radim Krčmář <rkrcmar@redhat.com> wrote:

> 2014-11-24 11:40+0100, Jan Kiszka:
> > On 2014-11-24 11:12, Paolo Bonzini wrote:
> > > On 24/11/2014 05:36, nick wrote:
> > >> Greetings Again Gleb and others,
> > >> I am assuming in the code I am pasting below the fix me is obsolete now and I can remove it. :)
> > >> Cheers Nick
> > >> TP_printk("%s (0x%x)",
> > >> __print_symbolic(__entry->exception, kvm_trace_sym_exc),
> > >> /* FIXME: don't print error_code if not present */
> > >> __entry->has_error ? __entry->error_code : 0)
> > >> );
> > >>
> > >
> > > No, it's not obsolete, the idea is to print only
> > >
> > > %s
> > >
> > > instead of
> > >
> > > %s (0x%x)
> > >
> > > if __entry->has_error is false. I don't know the trace API well enough
> > > to know if that is possible.
> >
> > Last time I ran across such a scenario, it was not feasible and
> > essentially required separate tracepoints. But maybe Steven knows a trick.
>
> The format string has to be a string literal[1]; we could change it to
> allow expressions[2], but what we want is almost possible through a
> direct call to trace_seq_printf()[3].
>
> The raw result would look like
>
> #define __print(fmt, args...) ({ \
> const char *buf_start = trace_seq_buffer_ptr(p); \
> trace_seq_printf(p, fmt, args); \
> trace_seq_putc(p, '\0'); \
> buf_start; \
> })
>
> TP_printk("%s%s", [...],
> __entry->has_error ? __print("(0x%x)", __entry->error_code) : "")
>
> and would be acceptable if something __print-like made it into a ftrace
> helper[4]. (Userspace won't be able to nicely print it otherwise.)

You mean if we add something like a __print_conditional(cond, fmt, ...);

For this case you would have:

TP_printk("%s%s", [...],
__print_conditional(__entry->has_error, " (0x%x)", __entry->error_code));

Where __print_conditional() will return "" when "cond" is false, and
will return the formatted string otherwise.

That wouldn't be too hard to implement.

-- Steve

>
>
> ---
> 1: #define TP_printk(fmt, args...) fmt "\n", args
> 2: TP_printk(__entry->has_error ? "%s (0x%x)" : "%s", [...]
> 3: Already in scsi_dispatch_cmd_start or kvm_mmu_get_page tracepoints.
> 4: Like __print_hex or print_symbolic.