Date: Thu, 17 Jun 2010 12:38:08 +1000 From: Lawrence Stewart <lstewart@freebsd.org> To: Kostik Belousov <kostikbel@gmail.com> Cc: Matthew Fleming <mdf@FreeBSD.org>, src-committers@FreeBSD.org, Pawel Jakub Dawidek <pjd@FreeBSD.org>, John Baldwin <jhb@freebsd.org>, svn-src-all@FreeBSD.org, brde@optusnet.com.au, svn-src-head@FreeBSD.org Subject: Re: svn commit: r209119 - head/sys/sys Message-ID: <4C198A90.3060905@freebsd.org> In-Reply-To: <20100614104349.GF13238@deviant.kiev.zoral.com.ua> References: <201006130239.o5D2du3m086332@svn.freebsd.org> <20100613101025.GD1320@garage.freebsd.pl> <4C158B71.205@freebsd.org> <20100614085205.GD13238@deviant.kiev.zoral.com.ua> <4C1605A7.2000202@freebsd.org> <20100614104349.GF13238@deviant.kiev.zoral.com.ua>
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On 06/14/10 20:43, Kostik Belousov wrote:
> On Mon, Jun 14, 2010 at 08:34:15PM +1000, Lawrence Stewart wrote:
>> On 06/14/10 18:52, Kostik Belousov wrote:
>>> On Mon, Jun 14, 2010 at 11:52:49AM +1000, Lawrence Stewart wrote:
>>>> On 06/13/10 20:10, Pawel Jakub Dawidek wrote:
>>>>> On Sun, Jun 13, 2010 at 02:39:55AM +0000, Lawrence Stewart wrote:
>>>> [snip]
>>>>>>
>>>>>> Modified: head/sys/sys/pcpu.h
>>>>>> ==============================================================================
>>>>>> --- head/sys/sys/pcpu.h Sun Jun 13 01:27:29 2010 (r209118)
>>>>>> +++ head/sys/sys/pcpu.h Sun Jun 13 02:39:55 2010 (r209119)
>>>>>> @@ -106,6 +106,17 @@ extern uintptr_t dpcpu_off[];
>>>>>> #define DPCPU_ID_GET(i, n) (*DPCPU_ID_PTR(i, n))
>>>>>> #define DPCPU_ID_SET(i, n, v) (*DPCPU_ID_PTR(i, n) = v)
>>>>>>
>>>>>> +/*
>>>>>> + * Utility macros.
>>>>>> + */
>>>>>> +#define DPCPU_SUM(n, var, sum) \
>>>>>> +do { \
>>>>>> + (sum) = 0; \
>>>>>> + u_int i; \
>>>>>> + CPU_FOREACH(i) \
>>>>>> + (sum) += (DPCPU_ID_PTR(i, n))->var; \
>>>>>> +} while (0)
>>>>>
>>>>> I'd suggest first swapping variable declaration and '(sum) = 0;'.
>>>>> Also using 'i' as a counter in macro can easly lead to name collision.
>>>>> If you need to do it, I'd suggest '_i' or something.
>>>>
>>>> Given that the DPCPU variable name space is flat and variable names have
>>>> to be unique, perhaps something like the following would address the
>>>> concerns raised?
>>>>
>>>> #define DPCPU_SUM(n, var, sum) \
>>>> do { \
>>>> u_int _##n##_i; \
>>>> (sum) = 0; \
>>>> CPU_FOREACH(_##n##_i) \
>>>> (sum) += (DPCPU_ID_PTR(_##n##_i, n))->var; \
>>>> } while (0)
>>>
>>> You do not have to jump through this. Mostly by convention, in our kernel
>>> sources, names with "_" prefix are reserved for the infrastructure (cannot
>>> say implementation). I think it is quite safe to use _i for the iteration
>>> variable.
>>>
>>> As an example of this, look at sys/sys/mount.h, implementation of
>>> VFS_NEEDGIANT, VFS_LOCK_GIANT etc macros. They do use gcc ({}) extension
>>> to provide function-like macros, but this is irrelevant. Or, look at
>>> the VFS_ASSERT_GIANT that is exactly like what you need.
>>
>> Ok cool, thanks for the info and pointers (I didn't know about the ({})
>> extension or that "_" prefix was definitely reserved). I'm happy to use
>> _i. Does the following diff against head look suitable to commit?
>>
>> --- a/sys/sys/pcpu.h Sun Jun 13 02:39:55 2010 +0000
>> +++ b/sys/sys/pcpu.h Mon Jun 14 20:12:27 2010 +1000
>> @@ -111,10 +111,10 @@
>> */
>> #define DPCPU_SUM(n, var, sum) \
>> do { \
>> + u_int _i; \
>> (sum) = 0; \
>> - u_int i; \
>> - CPU_FOREACH(i) \
>> - (sum) += (DPCPU_ID_PTR(i, n))->var; \
>> + CPU_FOREACH(_i) \
>> + (sum) += (DPCPU_ID_PTR(_i, n))->var; \
>> } while (0)
>
> You might want to introduce local accumulator to prevent several evaluations
> of sum, to avoid possible side-effects. Then, after, the loop, do single
> asignment to the the sum.
>
> Or, you could ditch the sum at all, indeed using ({}) and returning the
> result. __typeof is your friend to select proper type of accumulator.
So, something like this?
#define DPCPU_SUM(n, var) __extension__ \
({ \
u_int _i; \
__typeof((DPCPU_PTR(n))->var) sum; \
\
sum = 0; \
CPU_FOREACH(_i) { \
sum += (DPCPU_ID_PTR(_i, n))->var; \
} \
sum; \
})
Which can be used like this:
totalss.n_in = DPCPU_SUM(ss, n_in);
I've tested the above and it works. I also prefer the idea of having
DPCPU_SUM return the sum so that you can do "var = DPCPU_SUM(...)". My
only concern with this method is that the caller no longer has the
choice to make the sum variable a larger type to avoid overflow. It
would be nice to be able to have the DPCPU vars be uint32_t but be able
to sum them into a uint64_t accumulator for example. Perhaps this isn't
really an issue though... I'm not sure.
Thoughts?
Cheers,
Lawrence
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