linux-kernel - Re: Another Obsolete Fix me in trace.h?

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Message-ID: <20141124161901.6266925c@gandalf.local.home>
Date:	Mon, 24 Nov 2014 16:19:01 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	Jan Kiszka <jan.kiszka@...mens.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	nick <xerofoify@...il.com>, gleb@...nel.org, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Another Obsolete Fix me in trace.h?

On Mon, 24 Nov 2014 22:00:01 +0100
Radim Krčmář <rkrcmar@...hat.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 I am assuming in the code I am pasting below the fix me is obsolete now and I can remove it.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.

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