Re: [RFC PATCH bpf-next v2 4/4] error-injection: Support fault injection framework

[Masami Hiramatsu <mhiramat@kernel.org>]

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.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>