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

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Date:	Fri, 19 Dec 2008 11:16:32 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mike Travis <travis@....com>
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


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
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