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From: Denys Vlasenko <vda.linux@googlemail.com>
Date: Fri, 30 Jun 2017 20:50:45 +0200
Message-ID: <CAK1hOcOpeEG9V4VZh3QC11y7kchrfethuW4_Pg7N5FaVfP9R7g@mail.gmail.com>
Subject: Re: [PATCH] selftests: ftrace: Use md5sum to take less time of
 checking logs
To: Masami Hiramatsu <mhiramat@kernel.org>
Cc: linux-kselftest@vger.kernel.org, shuah@kernel.org,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Jun 27, 2017 at 12:28 PM, Masami Hiramatsu <mhiramat@kernel.org> wrote:
> Use md5sum so that it takes less time of checking
> trace logs update. Since busybox tail/cat takes too
> long time to read the trace log, this uses md5sum
> to check whether trace log is updated or not.
>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
>  .../test.d/ftrace/func_traceonoff_triggers.tc      |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/ftrace/test.d/ftrace/func_traceonoff_triggers.tc b/tools/testing/selftests/ftrace/test.d/ftrace/func_traceonoff_triggers.tc
> index 9cf3852..7a9ab4f 100644
> --- a/tools/testing/selftests/ftrace/test.d/ftrace/func_traceonoff_triggers.tc
> +++ b/tools/testing/selftests/ftrace/test.d/ftrace/func_traceonoff_triggers.tc
> @@ -103,11 +103,11 @@ if [ $on != "0" ]; then
>      fail "Tracing is not off"
>  fi
>
> -line1=`cat trace | tail -1`
> +csum1=`md5sum trace`
>  sleep $SLEEP_TIME
> -line2=`cat trace | tail -1`
> +csum2=`md5sum trace`

If you replace that with "tail -1 <trace", this code
in busybox will be used:

                        off_t current = lseek(fd, 0, SEEK_END);
                        if (current > 0) {
...
#if 1 /* This is technically incorrect for *LONG* strings, but very useful */
                                /* Optimizing count-lines case if the
file is seekable.
                                 * We assume the lines are <64k.
                                 * (Users complain that tail takes too long
                                 * on multi-gigabyte files) */
                                off = (count | 0xf); /* for small
counts, be more paranoid */
                                if (off > (INT_MAX / (64*1024)))
                                        off = (INT_MAX / (64*1024));
                                current -= off * (64*1024);
                                if (current < 0)
                                        current = 0;
                                xlseek(fd, current, SEEK_SET);
#endif

Example: "tail -1 10GB_FILE" seeks back ~1 mbyte and looks for the last line:

lseek(3, 0, SEEK_END)                   = 10991787403
lseek(3, 10990804363, SEEK_SET)         = 10990804363
read(3, "\0\0020d\340\326\0\0\0\0020e\6\364\0\0\0\0020eMI\0\0\0\0020el\210\0\0"...,
8192) = 8192
read(3, "\0\0021\"p\321\0\0\0\0021\"\216\31\0\0\0\0021\"\254q\0\0\0\0021\"\367u\0\0"...,
8192) = 8192
read(3, "\0\0021\342\216\302\0\0\0\0021\342\2243\0\0\0\0021\342\266T\0\0\0\0021\342\326\v\0\0"...,
8192) = 8192
...
read(3, "\0\0\2\211\3272\211\0\0\0\2\211\3277\235\0\0\0\2\211\327mx\0\0\0\2\211\327\224\22\0"...,
11841) = 11841
read(3, "\0\2\212\357\366\306\0\0\0\2\212\360`\232\0\0\0\2\212\360w\262\0\0\0\2\212\360\354\365\0\0"...,
12512) = 12512
read(3, "\0\2\214uL\3\0\0\0\2\214unc\0\0\0\2\214w_\373\0\0\0\2\214wn\6\0\0"...,
11176) = 8424
read(3, "", 2752)                       = 0