linux-kernel - Re: [RFD 4/9] Make total

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Date:	Wed, 28 Sep 2011 12:27:41 -0300
From:	Glauber Costa <glommer@...allels.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Martin Schwidefsky <schwidefsky@...ibm.com>,
	<linux-kernel@...r.kernel.org>, <paul@...lmenage.org>,
	<lizf@...fujitsu.com>, <daniel.lezcano@...e.fr>,
	<jbottomley@...allels.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [RFD 4/9] Make total_forks per-cgroup

On 09/28/2011 07:35 AM, Peter Zijlstra wrote:
> On Wed, 2011-09-28 at 10:13 +0200, Martin Schwidefsky wrote:
>> On Wed, 28 Sep 2011 00:00:37 +0200
>> Peter Zijlstra<a.p.zijlstra@...llo.nl>  wrote:
>>
>>> On Fri, 2011-09-23 at 19:20 -0300, Glauber Costa wrote:
>>>> @@ -1039,6 +1035,8 @@ static void posix_cpu_timers_init(struct task_struct *tsk)
>>>>          INIT_LIST_HEAD(&tsk->cpu_timers[2]);
>>>>   }
>>>>
>>>> +struct task_group *task_group(struct task_struct *p);
>>>
>>> That doesn't appear to be actually used in this file..
>>>
>>> Also, since there's already a for_each_possible_cpu() loop in that
>>> proc/stat function, would it yield some code improvement to make
>>> total_forks a cpu_usage_stat?
>>>
>>> I guess the whole cputime64_t crap gets in the way of that being
>>> natural...
>>>
>>> We could of course kill off the cputime64_t thing, its pretty pointless
>>> and its a u64 all over the board. I think Martin or Heiko created this
>>> stuff (although I might be wrong, my git tree doesn't go back that far).
>>
>> The reason to introduce cputime_t has been that different architecture
>> needed differently sized integers for their respective representation
>> of cputime. On x86-32 the number of ticks is recorded in a u32, on s390
>> we needed a u64 for the cpu timer values. cputime64_t is needed for
>> cpustat and other sums of cputime that would overflow a cputime_t
>> (in particular on x86-32 with the u32 cputime_t and the u64 cputime64_t).
>>
>> Now we would convert everything to u64 but that would cause x86-32 to
>> use 64-bit arithmetic for the tick counter. If that is acceptable I
>> can't say.
>
> Right, so the main point was about cputime64_t, we might as well use a
> u64 for that throughout and ditch the silly cputime64_$op() accessors
> and write normal code.
>
> But even if cputime_t differs between 32 and 64 bit machines, there is
> no reason actually use cputime_add(), C can do this.
>
> The only reason to use things like cputime_add() is if you use a non
> simple type, but that doesn't seem to be the case.
>
> So I think we can simplify the code lots by doing away with cputime64_t
> and all the cputime_*() functions. We can keep cputime_t, or we can use
> unsigned long, which I think will end up doing pretty much the same.
for cputime64, I'm with you here. (so far)
>
> That is, am I missing some added value of all this cputime*() foo?

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