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Date:   Wed, 25 Oct 2017 22:08:58 +0800
From:   "Du, Changbin" <changbin.du@...el.com>
To:     changbin.du@...el.com
Cc:     rostedt@...dmis.org, mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing: Allocate mask_str buffer dynamically

On Wed, Oct 25, 2017 at 07:24:36PM +0800, changbin.du@...el.com wrote:
> From: Changbin Du <changbin.du@...el.com>
> 
> The default NR_CPUS can be very large, but actual possible nr_cpu_ids
> usually is very small. For my x86 distribution, the NR_CPUS is 8192 and
> nr_cpu_ids is 4. About 2 pages are wasted.
> 
> Most machines don't have so many CPUs, so define a array with NR_CPUS
> just wastes memory. So let's allocate the buffer dynamically when need.
> 
> Signed-off-by: Changbin Du <changbin.du@...el.com>
> ---
>  kernel/trace/trace.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 752e5da..d1b3f11 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -4184,19 +4184,18 @@ static const struct file_operations show_traces_fops = {
>   */
>  static DEFINE_MUTEX(tracing_cpumask_update_lock);
>  
> -/*
> - * Temporary storage for the character representation of the
> - * CPU bitmask (and one more byte for the newline):
> - */
> -static char mask_str[NR_CPUS + 1];
> -
>  static ssize_t
>  tracing_cpumask_read(struct file *filp, char __user *ubuf,
>  		     size_t count, loff_t *ppos)
>  {
>  	struct trace_array *tr = file_inode(filp)->i_private;
> +	static char *mask_str;
ah, need remove 'static'.

>  	int len;
>  
> +	mask_str = kmalloc(nr_cpu_ids + 1, GFP_KERNEL);
> +	if (!mask_str)
> +		return -ENOMEM;
> +
>  	mutex_lock(&tracing_cpumask_update_lock);
>  
>  	len = snprintf(mask_str, count, "%*pb\n",
> @@ -4205,10 +4204,12 @@ tracing_cpumask_read(struct file *filp, char __user *ubuf,
>  		count = -EINVAL;
>  		goto out_err;
>  	}
> -	count = simple_read_from_buffer(ubuf, count, ppos, mask_str, NR_CPUS+1);
> +	count = simple_read_from_buffer(ubuf, count, ppos,
> +					mask_str, nr_cpu_ids+1);
>  
>  out_err:
>  	mutex_unlock(&tracing_cpumask_update_lock);
> +	kfree(mask_str);
>  
>  	return count;
>  }
> -- 
> 2.7.4
> 

-- 
Thanks,
Changbin Du

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