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 5:27 PM, Attilio Rao <attilio at freebsd.org> wrote:
> 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

BTW, the mtx_sysuninit() introduction can be avoided by using this other trick:
#define MTX_SYSINIT(name, mtx, desc, opts)                              \
        static struct mtx_args name##_args = {                          \
                (mtx),                                                  \
                (desc),                                                 \
                (opts)                                                  \
        };                                                              \
        SYSINIT(name##_mtx_sysinit, SI_SUB_LOCK, SI_ORDER_MIDDLE,       \
            mtx_sysinit, &name##_args);                                 \
        SYSUNINIT(name##_mtx_sysuninit, SI_SUB_LOCK, SI_ORDER_MIDDLE,   \
            _mtx_destroy, __DEVOLATILE(void *, &(mtx)->mtx_lock))

I'm just not sure that I would like the use of __DEVOLATILE() even if
it would help in this case when introducing MTX_SYSINIT_UNSHARE()
because we will just need to reuse the same code.

Also, the more I think about this the more I feel convinced that
mtxlock2mtx() should be static in kern_mutex.c. I can simply add a
note to _mutex.h as a reminder for it.

Attilio


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