Re: [RFC] An idea for general kernel post-processing automation in FreeBSD

On 5/23/23 6:30 AM, Mark Johnston wrote:
> On Tue, May 23, 2023 at 08:00:53AM +0200, Hans Petter Selasky wrote:
>> Hi Warner,
>>
>> When you make systems and use them, you get bound by them. If a system makes
>> a certain solution advantageous, it gets chosen, independent if it is a good
>> solution or not.
>>
>> The reason for our disagreement is simply this:
>>
>> You are thinking like a politican and want to be popular in the current
>> system of FreeBSD.
>>
>> I'm thinking like a mathematican regardless of what makes me popular in the
>> current system of FreeBSD.
>>
>>
>> Why not state that from the start? Implementing Quick Sort in qsort() and
>> using that everywhere is a political decision.
>>
>> This is not a technical fight, it is a political fight.
> 
> I don't agree.  From my point of view, Warner's position is the
> pragmatic one.  We have perhaps 2% Linux's number of active
> developers[*], but we are relatively much larger in terms of
> performance, code complexity, size, etc..
> 
> Layering and simplicity of design are some of the main tools we have to
> counteract this imbalance.  It helps reduce the amount of time
> developers spend on bugs that aren't directly related to what they are
> doing.  It helps us think about and predict the behaviour of the system
> using only intuition.  This tradeoff can mean that we do not provide the
> best possible performance in all cases, but that's often a reasonable
> tradeoff in this rather non-mathematical world where we do not have
> infinite resources.
> 
> The existing SYSINIT bubblesort is a good example of this tradeoff.  At
> the time it was written, it made sense to choose a simple, "good enough"
> solution and move on.  Even now, this simple solution just works and is
> perfectly acceptable on the vast majority of systems where FreeBSD is
> deployed.
> 
> I see this attitude reflected in Warner's and others' replies, and I
> agree with it.  I suspect that Warner, rather than wanting to be popular
> per se, is replying in his own self-interest, which is to spend zero
> time debugging anything that might break if we start doing extra work at
> compile time to sort linker sets.
> 
> It could be that some specific use-case will make your proposal more
> attractive.  I don't mean to suggest that the topic should be closed
> forever.  So far though, having read the thread and D40193, I'm not
> really sold.
> 
> [*] I'm sure this number can vary wildly depending on how you define it;
> my impression from reading Linux lists for a while is just that we have
> way fewer people who understand core pieces of the system.

+1.

Also, the concern over the runtime for the potential worst-case for qsort
seems _way_ overblown compared to the actual unsorted arrays one will get
out of the linker.  The only possible downside of qsort that matters in
this case is the stack usage.

When Colin first mentioned this, my initial thought if we wanted to be
cute would be to use a post-link tool like kldxref to sort the linker set,
but such a tool would actually be rather painful to write and fraught
with all the peril of the current kldxref tool (which is not yet a cross
tool).  The patch to use qsort by comparison is very small and easy to
reason about and maintain.

-- 
John Baldwin