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

[Alexei Starovoitov]

On 12/27/17 5:38 PM, Masami Hiramatsu wrote:
> On Wed, 27 Dec 2017 14:49:46 -0800
> Alexei Starovoitov <ast@xxxxxx> wrote:
>
> > On 12/27/17 12:09 AM, Masami Hiramatsu wrote:
> > > On Tue, 26 Dec 2017 18:12:56 -0800
> > > Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> 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@xxxxxxxxxx>
> > > > > ---
> > > > >  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.