linux-kernel - Re: [PATCH 2/3] tracing: instruct KCOV not to track tracing files

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Date:   Fri, 30 Nov 2018 10:22:14 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Anders Roxell <anders.roxell@...aro.org>
Cc:     mingo@...hat.com, keescook@...omium.org, catalin.marinas@....com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 2/3] tracing: instruct KCOV not to track tracing files

On Fri, 30 Nov 2018 16:09:35 +0100
Anders Roxell <anders.roxell@...aro.org> wrote:

> When we have KCOV enabled and running ftrace startup tests we end up in
> a softlockup. Kcov and ftrace tracing each other makes it really slow:
> 
> [  275.141388] Testing tracer wakeup_dl:  PASSED
> [  304.738345] Testing tracer function_graph:
> [  716.236822] watchdog: BUG: soft lockup - CPU#0 stuck for 21s! [ksoftirqd/0:9]
> 
> Rework so that we don't let KCOV look at tracing files. Could probably
> be more selective here, but in in general letting KCOV and ftrace check
> each isn't the best idea.
> 
> Co-developed-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Anders Roxell <anders.roxell@...aro.org>
> ---
>  kernel/trace/Makefile | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
> index f81dadbc7c4a..c7c73b976103 100644
> --- a/kernel/trace/Makefile
> +++ b/kernel/trace/Makefile
> @@ -6,6 +6,11 @@ ifdef CONFIG_FUNCTION_TRACER
>  ORIG_CFLAGS := $(KBUILD_CFLAGS)
>  KBUILD_CFLAGS = $(subst $(CC_FLAGS_FTRACE),,$(ORIG_CFLAGS))
>  
> +# If instrumentation of this dir is enabled, the function tracer gets really
> +# slow. Probably could be more selective here, but note that files related
> +# to tracing.shouldn't be traced anyway.
> +KCOV_INSTRUMENT		:= n
> +

The entire directory is also set to not be traced by function tracing,
which also is a bit overkill, as there's functions in this directory
that can (and probably should) be.

Acked-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

-- Steve

>  ifdef CONFIG_FTRACE_SELFTEST
>  # selftest needs instrumentation
>  CFLAGS_trace_selftest_dynamic.o = $(CC_FLAGS_FTRACE)