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From:   Sven Schnelle <svens@linux.ibm.com>
To:     Yinan Liu <yinan@linux.alibaba.com>
Cc:     rostedt@goodmis.org, peterz@infradead.org,
        mark-pk.tsai@mediatek.com, mingo@redhat.com,
        linux-kernel@vger.kernel.org, hca@linux.ibm.com,
        linux-s390@vger.kernel.org
Subject: Re: [PATCH v8] scripts: ftrace - move the sort-processing in
 ftrace_init
References: <20210911135043.16014-1-yinan@linux.alibaba.com>
        <20211212113358.34208-1-yinan@linux.alibaba.com>
        <20211212113358.34208-2-yinan@linux.alibaba.com>
Date:   Fri, 21 Jan 2022 10:46:36 +0100
In-Reply-To: <20211212113358.34208-2-yinan@linux.alibaba.com> (Yinan Liu's
        message of "Sun, 12 Dec 2021 19:33:58 +0800")
Message-ID: <yt9dee51ctfn.fsf@linux.ibm.com>
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Hi Yinan,

Yinan Liu <yinan@linux.alibaba.com> writes:

> When the kernel starts, the initialization of ftrace takes
> up a portion of the time (approximately 6~8ms) to sort mcount
> addresses. We can save this time by moving mcount-sorting to
> compile time.
>
> Signed-off-by: Yinan Liu <yinan@linux.alibaba.com>
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: kernel test robot <oliver.sang@intel.com>
> ---
>  kernel/trace/ftrace.c   |  11 +++-
>  scripts/Makefile        |   6 +-
>  scripts/link-vmlinux.sh |   6 +-
>  scripts/sorttable.c     |   2 +
>  scripts/sorttable.h     | 120 +++++++++++++++++++++++++++++++++++++++-
>  5 files changed, 137 insertions(+), 8 deletions(-)

while i like the idea, this unfortunately breaks ftrace on s390. The
reason for that is that the compiler generates relocation entries for
all the addresses in __mcount_loc. During boot, the s390 decompressor
iterates through all the relocations and overwrites the nicely
sorted list between __start_mcount_loc and __stop_mcount_loc with
the unsorted list because the relocations entries are not adjusted.

Of course we could just disable that option, but that would make us
different compared to x86 which i don't like. Adding code to sort the
relocation would of course also fix that, but i don't think it is a good
idea to rely on the order of relocations.

Any thoughts how a fix could look like, and whether that could also be a
problem on other architectures?

Thanks
Sven