linux-kernel - Re: [PATCH 4/8] cpumask: convert kernel trace functions

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Date:	Fri, 19 Dec 2008 08:54:40 -0800
From:	Mike Travis <travis@....com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/8] cpumask: convert kernel trace functions

Steven Rostedt wrote:
> On Fri, 19 Dec 2008, Mike Travis wrote:
> 
>> Impact: Reduce memory usage, use new cpumask API.
>>
>> Convert kernel trace functions to use struct cpumask.
>>
> 
> [..]
> 
>>  #define for_each_buffer_cpu(buffer, cpu)		\
>> -	for_each_cpu_mask(cpu, buffer->cpumask)
>> +	for_each_cpu(cpu, buffer->cpumask)
>>  
>>  #define TS_SHIFT	27
>>  #define TS_MASK		((1ULL << TS_SHIFT) - 1)
>> @@ -262,7 +262,7 @@ struct ring_buffer {
>>  	unsigned			pages;
>>  	unsigned			flags;
>>  	int				cpus;
>> -	cpumask_t			cpumask;
>> +	cpumask_var_t			cpumask;
>>  	atomic_t			record_disabled;
>>  
>>  	struct mutex			mutex;
>> @@ -453,6 +453,9 @@ struct ring_buffer *ring_buffer_alloc(un
>>  	if (!buffer)
>>  		return NULL;
>>  
>> +	if (!alloc_cpumask_var(&buffer->cpumask, GFP_KERNEL))
>> +		goto fail_free_buffer;
>> +
> 
> 
> How does this save memory if we just allocate a cpumask var everytime
> we allocate a ring buffer?  Is cpumask_var_t a mask of possible CPUS and 
> not NR_CPUS?
> 
> Otherwise, I see this as just adding one extra pointer.
> 
> -- Steve

Hi Steve,

Yes, eventually, the cpumask_var_t will be allocated based on cpumask_size()
which will become BITS_TO_LONG(nr_cpu_ids) instead of BITS_TO_LONGS(NR_CPUS)
as soon as the kernel becomes "cpumask" clean.  (clean being that it ignores
all bits >= nr_cpu_ids.)

Note that on small NR_CPUS count systems, cpumask_var_t is a static array
and no allocation happens, so it only kicks in when CONFIG_CPUMASK_OFFSTACK=y.

Thanks,
Mike
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