linux-kernel - ftrace on MIPS/ath79

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Date:   Fri, 23 Mar 2018 23:20:10 +0100
From:   Matthias Schiffer <mschiffer@...verse-factory.net>
To:     rostedt@...dmis.org, Ralf Baechle <ralf@...ux-mips.org>
Cc:     linux-mips@...ux-mips.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: ftrace on MIPS/ath79

Hi,
I'm currently trying to debug a performance bottleneck on low-end ath79
hardware running OpenWrt/LEDE, but it seems that ftrace is not working
correctly on these systems. I have tried this with recent 4.4.y and 4.9.y
with similar results; unfortunately, switching to a newer kernel is not
easily possible on this hardware at the moment. Please let me know if there
are any known issues or patches that I should backport.

There seem to be two separate issues:

1) Building with CONFIG_DYNAMIC_FTRACE leads to a kernel panic as soon as
kernel modules are loaded (logs attached).

2) function_graph tracer does not show anything useful: the trace output
looks like what was reported in [1]. Building with
CONFIG_FUNCTION_GRAPH_TRACER leads to a completely empty
trace_stat/function0 (except for the header); profiling is working as
expected when CONFIG_FUNCTION_GRAPH_TRACER is disabled.

I would be thankful for any pointers that might help me to make this work.

Kind regards,
Matthias

[1] https://www.linux-mips.org/archives/linux-mips/2014-11/msg00295.html

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