linux-kernel - Re: [PATCH 10/10] fault injection: inject faults in new/rare callchains

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Date:	Mon, 8 Aug 2016 16:07:39 +0200
From:	Vegard Nossum <vegard.nossum@...cle.com>
To:	Akinobu Mita <akinobu.mita@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/10] fault injection: inject faults in new/rare
 callchains

Hi,

On 08/08/2016 03:54 PM, Akinobu Mita wrote:
> 2016-08-04 0:05 GMT+09:00 Vegard Nossum <vegard.nossum@...cle.com>:
>> Before this patch, fault injection uses a combination of randomness and
>> frequency to determine where to inject faults. The problem with this is
>> that code paths which are executed very rarely get proportional amounts
>> of faults injected.
>>
>> A better heuristic is to look at the actual callchain leading up to the
>> possible failure point; if we see a callchain that we've never seen up
>> until this point, chances are it's a rare one and we should definitely
>> inject a fault here (since we might not get the chance again later).
>>
>> This uses a probabilistic set structure (similar to a bloom filter) to
>> determine whether we have seen a particular callchain before by hashing
>> the stack trace and atomically testing/setting a bit corresponding to
>> the current callchain.
[...]

>> +config FAULT_INJECTION_AT_NEW_CALLSITES
>> +       bool "Inject fault the first time at a new callsite"
>
> Isn't it better to make a run time configurable option instead of the
> build option?

I prefer a build option personally since it keeps the code simple (you
don't have to dynamically allocate the bitmap of known callchains, for
example). I figured most people using fault injection would enable the
new option while still allowing others to keep the current behaviour
if they really want to.

If you prefer a run-time option I can submit a new version.

Thanks,


Vegard