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Date:   Wed, 29 Nov 2017 22:12:09 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     changbin.du@...el.com
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v3] tracing: Allocate mask_str buffer dynamically

On Wed, 29 Nov 2017 12:42:45 +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.
> 
> The exact buffer size should be:
>   DIV_ROUND_UP(nr_cpu_ids, 4) + nr_cpu_ids/32 + 2;
> 
> Example output:
>   ff,ffffffff

Um, what if there's more than 64 CPUs, where I have booted several
boxes that have more. There's going to be more than 1 comma.

> 
> With this change, the mutext tracing_cpumask_update_lock also can be
> removed now, which was used to protect mask_str.
> 
> Signed-off-by: Changbin Du <changbin.du@...el.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> 
> ---
> v3:
>   - remove tracing_cpumask_update_lock which was used to protect mask_str. (Rostedt)
> v2:
>   - remove 'static' declaration.
>   - fix buffer size.
> ---
>  kernel/trace/trace.c | 29 +++++++++--------------------
>  1 file changed, 9 insertions(+), 20 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 73e67b6..6750d05 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -4178,37 +4178,30 @@ static const struct file_operations show_traces_fops = {
>  	.llseek		= seq_lseek,
>  };
>  
> -/*
> - * The tracer itself will not take this lock, but still we want
> - * to provide a consistent cpumask to user-space:
> - */
> -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;
> +	char *mask_str;
>  	int len;
>  
> -	mutex_lock(&tracing_cpumask_update_lock);
> +	/* Bitmap, ',' and two more bytes for the newline and '\0'. */
> +	len = DIV_ROUND_UP(nr_cpu_ids, 4) + nr_cpu_ids/32 + 2;

This is broken. Instead do:

	len = snprintf(NULL, 0, "%*pb\n",
		       cpumask_pr_args(tr->tracing_cpumask)) + 1;

	mask_str = kmalloc(len, GFP_KERNEL);
	[..]
	len = snprintf(mask_str, len, "%*pb\n",
		       cpumask_pr_args(tr->tracing_cpumask));

-- Steve

> +	mask_str = kmalloc(len, GFP_KERNEL);
> +	if (!mask_str)
> +		return -ENOMEM;
>  
> -	len = snprintf(mask_str, count, "%*pb\n",
> +	len = snprintf(mask_str, len, "%*pb\n",
>  		       cpumask_pr_args(tr->tracing_cpumask));
>  	if (len >= count) {
>  		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, len);
>  
>  out_err:
> -	mutex_unlock(&tracing_cpumask_update_lock);
> +	kfree(mask_str);
>  
>  	return count;
>  }
> @@ -4228,8 +4221,6 @@ tracing_cpumask_write(struct file *filp, const char __user *ubuf,
>  	if (err)
>  		goto err_unlock;
>  
> -	mutex_lock(&tracing_cpumask_update_lock);
> -
>  	local_irq_disable();
>  	arch_spin_lock(&tr->max_lock);
>  	for_each_tracing_cpu(cpu) {
> @@ -4252,8 +4243,6 @@ tracing_cpumask_write(struct file *filp, const char __user *ubuf,
>  	local_irq_enable();
>  
>  	cpumask_copy(tr->tracing_cpumask, tracing_cpumask_new);
> -
> -	mutex_unlock(&tracing_cpumask_update_lock);
>  	free_cpumask_var(tracing_cpumask_new);
>  
>  	return count;

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