svn commit: r241889 - in user/andre/tcp_workqueue/sys: arm/arm cddl/compat/opensolaris/kern cddl/contrib/opensolaris/uts/common/dtrace cddl/contrib/opensolaris/uts/common/fs/zfs ddb dev/acpica dev/...

[Attilio Rao]

On Sat, Oct 27, 2012 at 3:35 PM, Attilio Rao <attilio at freebsd.org> wrote:
> On 10/26/12, Andre Oppermann <andre at freebsd.org> wrote:
>> On 26.10.2012 08:27, Attilio Rao wrote:
>>> On Thu, Oct 25, 2012 at 10:03 PM, Andre Oppermann <andre at freebsd.org>
>>> wrote:
>>>> On 25.10.2012 18:21, Attilio Rao wrote:
>>>>> Did you see my (and also Jeff) objection to your proposal about this?
>>>>> You are deliberating ignoring this?
>>>>
>>>>
>>>> Well, I'm allowed to have a different opinion, am I?  I'm not ignoring
>>>> your objection in the sense as I'm not trying to commit any of this to
>>>> HEAD while it is disputed.
>>>>
>>>> Mind you this whole conversation was started because I was trying to
>>>> solve
>>>> a problem with unfortunate cache line sharing for global mutexes in the
>>>> kernel .bss section on my *personal* svn branch.
>>>
>>> Andre,
>>> I'm sorry if you felt I was being harsh or confrontative. This was
>>> really not my intention and I apologize.
>>
>> I apologize too for being a bit difficult and taking some time understand
>> the differences in __aligned() regarding padding behavior.
>>
>>> Said that, I fail in seeing a proper technical discussion on your side
>>> on how what I propose is "overdoing it" or how do you plan to address
>>> the concerns people are raising with your proposal of bumping all lock
>>> sizes indiscriminately.
>>
>> I'm wary of micro-optimizing and generally prefer clean and for a
>> reader obvious approaches.  That said my assumption on the distribution
>> of mutex use cases in the kernel was wrong.  By counting from a grep
>> it seems that about half of the mutexes could possibly benefit from
>> being padded and the other half doesn't because it is in structures
>> and next to its data.
>
> Besides that, you are likely misunderstanding something about what I
> propose: what I'm proposing is completely transparent to developers.
> You will just need to declare a mtx like:
>
> struct mtx_unshare Giant;
>
> and then you can use the mtx(9) interface on it without any issue. I
> don't see how this is less clean than what you propose. It just
> enables the alignment/padding on a selection basis rather than
> indiscriminately.
>
>>> However, here is the first half of the patch I'd like to see in:
>>> http:///www.freebsd.org/~attilio/mtx_decoupled.patch
>>  >
>>> This is just the part to give the ability to crunch different
>>> structures to the mtx KPI. Please note that from the users perspective
>>> the mtx KPI remains absolutely the same, so there is theoretically no
>>> KPI discontinuity, the support is absolutely transparent.
>>
>> This seems rather complicated.  Instead of mtxlock2mtx() wouldn't
>> __containerof() work just as well?  The __DEVOLATILE() looks a bit
>> dangerous.  Are you sure the compiler won't reorder things it should
>> not?
>
> What do you mean with "rather complicated"?
> For the users of the primitive nothing changes at all.
> For the people that might read the code it is pretty much
> self-explanatory, in particular if you know how lock classes work in
> our locking scheme. Maybe I can add a comment or two to clarify.

Here we go with further comments tweaks.
Also, in order to make it a complete NOP from KPI perspective I had to
change the way the mtx_assert() wrapper was implemented as in v1 it
wasn't correctly handling the const qualifier.
I think the result is better now and you should refer to this patch for reviews:
http://www.freebsd.org/~attilio/mtx_decoupled2.patch

Thanks,
Attilio


-- 
Peace can only be achieved by understanding - A. Einstein