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

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Message-ID: <4E833CAF.7010208@parallels.com>
Date:	Wed, 28 Sep 2011 12:26:39 -0300
From:	Glauber Costa <glommer@...allels.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	<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 05:13 AM, 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.
>
If we get rid of cputime64_t, it doesn't mean we need to get rid of 
cputime_t. Or am I missing the point here?

We would still have to call an analogous of cputime_to_cputime64 before 
using it, but I get that where it matters, gcc can even make it void.
And for all the rest, we do u64 math and typing and just get happy.

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