Subject: Re: [RFC PATCH bpf-next v2 4/4] error-injection: Support fault
 injection framework
To: Masami Hiramatsu <mhiramat@kernel.org>
References: <151427438796.32561.4235654585430455286.stgit@devbox>
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CC: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Josef Bacik <jbacik@fb.com>, <rostedt@goodmis.org>, <mingo@redhat.com>,
        <davem@davemloft.net>, <netdev@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <ast@kernel.org>, <kernel-team@fb.com>,
        <daniel@iogearbox.net>, <linux-btrfs@vger.kernel.org>,
        <darrick.wong@oracle.com>, Josef Bacik <josef@toxicpanda.com>,
        Akinobu Mita <akinobu.mita@gmail.com>
From: Alexei Starovoitov <ast@fb.com>
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Date: Wed, 27 Dec 2017 19:49:28 -0800
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On 12/27/17 5:38 PM, Masami Hiramatsu wrote:
> On Wed, 27 Dec 2017 14:49:46 -0800
> Alexei Starovoitov <ast@fb.com> wrote:
>
>> On 12/27/17 12:09 AM, Masami Hiramatsu wrote:
>>> On Tue, 26 Dec 2017 18:12:56 -0800
>>> Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
>>>
>>>> On Tue, Dec 26, 2017 at 04:48:25PM +0900, Masami Hiramatsu wrote:
>>>>> Support in-kernel fault-injection framework via debugfs.
>>>>> This allows you to inject a conditional error to specified
>>>>> function using debugfs interfaces.
>>>>>
>>>>> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
>>>>> ---
>>>>>  Documentation/fault-injection/fault-injection.txt |    5 +
>>>>>  kernel/Makefile                                   |    1
>>>>>  kernel/fail_function.c                            |  169 +++++++++++++++++++++
>>>>>  lib/Kconfig.debug                                 |   10 +
>>>>>  4 files changed, 185 insertions(+)
>>>>>  create mode 100644 kernel/fail_function.c
>>>>>
>>>>> diff --git a/Documentation/fault-injection/fault-injection.txt b/Documentation/fault-injection/fault-injection.txt
>>>>> index 918972babcd8..6243a588dd71 100644
>>>>> --- a/Documentation/fault-injection/fault-injection.txt
>>>>> +++ b/Documentation/fault-injection/fault-injection.txt
>>>>> @@ -30,6 +30,11 @@ o fail_mmc_request
>>>>>    injects MMC data errors on devices permitted by setting
>>>>>    debugfs entries under /sys/kernel/debug/mmc0/fail_mmc_request
>>>>>
>>>>> +o fail_function
>>>>> +
>>>>> +  injects error return on specific functions by setting debugfs entries
>>>>> +  under /sys/kernel/debug/fail_function. No boot option supported.
>>>>
>>>> I like it.
>>>> Could you document it a bit better?
>>>
>>> Yes, I will do in next series.
>>>
>>>> In particular retval is configurable, but without an example no one
>>>> will be able to figure out how to use it.
>>>
>>> Ah, right. BTW, as I pointed in the covermail, should we store the
>>> expected error value range into the injectable list? e.g.
>>>
>>> ALLOW_ERROR_INJECTION(open_ctree, -1, -MAX_ERRNO)
>>>
>>> And provide APIs to check/get it.
>>
>> I'm afraid such check would be too costly.
>> Right now we have only two functions marked but I expect hundreds more
>> will be added in the near future as soon as developers realize the
>> potential of such error injection.
>> All of ALLOW_ERROR_INJECTION marks add 8 byte overhead each to .data.
>> Multiple by 1k and we have 8k of data spent on marks.
>> If we add max/min range marks that doubles it for very little use.
>> I think marking function only is enough.
>
> Sorry, I don't think so.
> Even if it takes 16 bytes more for each points, I don't think it is
> any overhead for machines in these days. Even if so, we can provide
> a kconfig to reduce it.
> I mean, we are living in GB-order memory are, and it will be bigger
> in the future. Why we have to worry about hundreds of 16bytes memory
> pieces? It will take a few KB, and even if we mark thousands of
> functions, it never reaches 1MB, in GB memory pool. :)
>
> Of course, for many small-footprint embedded devices (like having
> less than 128MB memory), this feature can be a overhead. But they
> can cut off the table by kconfig.

I still disagree on wasting 16-byte * num_of_funcs of .data here.
The trade-off of usability vs memory just not worth it. Sorry.
Please focus on testing your changes instead.