linux-kernel - Re: [RFD 1/9] Change cpustat fields to an array.

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Message-ID: <4E836546.7010004@parallels.com>
Date:	Wed, 28 Sep 2011 15:19:50 -0300
From:	Glauber Costa <glommer@...allels.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	<linux-kernel@...r.kernel.org>, <paul@...lmenage.org>,
	<lizf@...fujitsu.com>, <daniel.lezcano@...e.fr>,
	<jbottomley@...allels.com>
Subject: Re: [RFD 1/9] Change cpustat fields to an array.

On 09/27/2011 06:00 PM, Peter Zijlstra wrote:
> On Fri, 2011-09-23 at 19:20 -0300, Glauber Costa wrote:
>>   /* Must have preemption disabled for this to be meaningful. */
>> -#define kstat_this_cpu __get_cpu_var(kstat)
>> +#define kstat_this_cpu this_cpu_ptr(task_group_kstat(current))
>
> This just lost you a debug check, the former would whinge when called
> without preemption, the new one wont. Its part of the this_cpu feature
> set to make debugging impossible.
>
>> +#else
>> +#define kstat_cpu(cpu) per_cpu(kstat, cpu)
>> +#define kstat_this_cpu (&__get_cpu_var(kstat))
>> +#endif
>>
>>   extern unsigned long long nr_context_switches(void);
>>
>> @@ -52,8 +62,8 @@ struct irq_desc;
>>   static inline void kstat_incr_irqs_this_cpu(unsigned int irq,
>>                                              struct irq_desc *desc)
>>   {
>> -       __this_cpu_inc(kstat.irqs[irq]);
>> -       __this_cpu_inc(kstat.irqs_sum);
>> +       kstat_this_cpu->irqs[irq]++;
>> +       kstat_this_cpu->irqs_sum++;
>
> It might be worth looking at the asm output of that, I think you made it
> worse, but I'm not quite sure how smart gcc is, it might just figure out
> what you meant.

I'd say leave it alone.
The biggest difference is that we don't have access to task_group(), or 
any of the fields in struct task_group. Because of that, we end up 
having to export a function to do the job of dealing with it.

Users inside sched.c won't have this problem. Outside of it, we'll add a 
call to some paths. True, mostly handle_irq paths, but I don't think 
that's what's going to kill us.

Now if we really really want to save it, we'd have to move struct 
task_group and its friends to a more visible location like a header...


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