linux-kernel - Re: [PATCH v2 2/2] selftests/ftrace: Add new test case which checks for optimized probes

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Date:   Tue, 25 Apr 2023 10:58:30 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
To:     Akanksha J N <akanksha@...ux.ibm.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, rostedt@...dmis.org,
        shuah@...nel.org
Subject: Re: [PATCH v2 2/2] selftests/ftrace: Add new test case which checks
 for optimized probes

Masami Hiramatsu wrote:
> On Tue, 18 Apr 2023 15:25:57 +0530
> Akanksha J N <akanksha@...ux.ibm.com> wrote:
> 
>> Add new test case kprobe_opt_types.tc which enables and checks
>> if each probe has been optimized in order to test potential issues with
>> optimized probes.
>> The '|| continue' is added with the echo statement to ignore errors that
>> are caused by trying to add kprobes to non probeable lines and continue
>> with the test.
>> Signed-off-by: Akanksha J N <akanksha@...ux.ibm.com>
>> ---
>>  .../ftrace/test.d/kprobe/kprobe_opt_types.tc  | 34 +++++++++++++++++++
>>  1 file changed, 34 insertions(+)
>>  create mode 100644 tools/testing/selftests/ftrace/test.d/kprobe/kprobe_opt_types.tc
>> 
>> diff --git a/tools/testing/selftests/ftrace/test.d/kprobe/kprobe_opt_types.tc b/tools/testing/selftests/ftrace/test.d/kprobe/kprobe_opt_types.tc
>> new file mode 100644
>> index 000000000000..54e4800b8a13
>> --- /dev/null
>> +++ b/tools/testing/selftests/ftrace/test.d/kprobe/kprobe_opt_types.tc
>> @@ -0,0 +1,34 @@
>> +#!/bin/sh
>> +# SPDX-License-Identifier: GPL-2.0-or-later
>> +# Copyright (C) 2023 Akanksha J N, IBM corporation
>> +# description: Register/unregister optimized probe
>> +# requires: kprobe_events
>> +
>> +case `uname -m` in
>> +x86_64)
>> +;;
>> +arm*)
>> +;;
>> +ppc*)
>> +;;
>> +*)
>> +  echo "Please implement other architecture here"
>> +  exit_unsupported
>> +esac
>> +
>> +DEFAULT=$(cat /proc/sys/debug/kprobes-optimization)
>> +echo 1 > /proc/sys/debug/kprobes-optimization
>> +for i in `seq 0 255`; do
>> +        echo  "p:testprobe $FUNCTION_FORK+${i}" > kprobe_events || continue
>> +        echo 1 > events/kprobes/enable || continue
>> +        (echo "forked")
>> +        PROBE_TYPE=$(cat /sys/kernel/debug/kprobes/list | grep $FUNCTION_FORK | awk '{print $4}' | awk '{print substr($0,2,length($0)-2)}')
> 
> I think we can make it simply;
> 
> PROBE=$(grep $FUNCTION_FORK /sys/kernel/debug/kprobes/list)
> 
>> +        echo 0 > events/kprobes/enable
>> +        echo > kprobe_events
>> +        if [ $PROBE_TYPE = "OPTIMIZED" ]; then
> 
> and
> 
> if echo $PROBE | grep -q OPTIMIZED; then
> 
>> +                echo "$DEFAULT" >  /proc/sys/debug/kprobes-optimization
>> +                exit_pass
>> +        fi
>> +done
>> +echo "$DEFAULT" >  /proc/sys/debug/kprobes-optimization
>> +echo "Done"
> 
> Hmm, this test does NOT return any error. It always returns success.

Good catch!

> I understand that optimization may not be possible within 256 bytes
> from the beginning of the function.

Is that true in practice? Looking at x86 and ppc64le, it looks like we 
will almost always be able to optimize at least one of the instructions 
within the first 256 bytes of kernel_clone(). That's one of the primary 
purposes of this test.

Are there valid reasons why we may not be able to optimize instructions?

> In that case, you can return
> "unresolved", and not echoing "Done" but the reason why it is
> unresolved.


- Naveen