Messages in this thread | | | Date | Tue, 12 Feb 2019 20:41:13 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCH 1/1] tracing: Fix event_trace_printk loss on printk_format. |
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On Tue, 12 Feb 2019 19:41:57 +0800 Mars Cheng <mars.cheng@mediatek.com> wrote:
> From: Andress Kuo <andress.kuo@mediatek.com> > > If fmt on event_trace_printk is not a constant, It means that something not > guaranteed, so the compiler optimizes the fmt out, and then the > __trace_printk_fmt section is not filled. if __trace_printk_fmt is not > filled, the kernel will not allocate the special buffers needed for the > event_trace_printk() and then not shown in the file output > sys/kernel/debug/tracing/print_formats. > > Adding a "__used" to the variable in the __trace_printk_fmt section on > event_trace_printk() will keep it around, even though it is set to NULL. > This will keep the string from being printed in the > sys/kernel/debug/tracing/printk_formats section. > > We can also refer to commit 3debb0a9ddb1 ("tracing: Fix trace_printk() > to print when not using bprintk()")that it had similar issue on path of > trace_printk().
Honestly, I don't even remember why that macro was created. I think it's a remnant from the creation of the trace events. I think the best solution is just to nuke it. It shouldn't be used anymore.
I'll dig a bit deeper into the history of that macro, but I'm thinking it shouldn't exist anymore.
[ /me returns from walking down memory lane ]
Wow, that brings back some wild memories. Yes, that macro must die, and I wish the history of it could die along with it ;-) The horror of the old ways I tell you. The event_trace_printk() was the original way we displayed events! No real formatting, no parsing my userspace tools. It was just a glamorized printk. It was called TRACE_FORMAT() which was deprecated by today's TRACE_EVENT().
That macro should have been removed by commit b8e65554d80b4.
Please just send a patch to delete that macro. Let's not be maintaining it. It gives me nightmares.
-- Steve
> > Signed-off-by: Andress Kuo <andress.kuo@mediatek.com> > Signed-off-by: Mars Cheng <mars.cheng@mediatek.com> > --- > include/linux/trace_events.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h > index 8a62731..26bd7c4 100644 > --- a/include/linux/trace_events.h > +++ b/include/linux/trace_events.h > @@ -551,7 +551,7 @@ extern int trace_define_field(struct trace_event_call *call, const char *type, > __trace_printk_check_format(fmt, ##args); \ > tracing_record_cmdline(current); \ > if (__builtin_constant_p(fmt)) { \ > - static const char *trace_printk_fmt \ > + static const char *trace_printk_fmt __used \ > __attribute__((section("__trace_printk_fmt"))) = \ > __builtin_constant_p(fmt) ? fmt : NULL; \ > \
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