linux-kernel - Re: [PATCH 3/6] tracing: tracing_stats_read() uses simple buffer instead of trace

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Date:	Wed, 20 Jan 2010 19:00:21 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/6] tracing: tracing_stats_read() uses simple buffer
	instead of trace_seq

On Tue, Jan 19, 2010 at 03:34:08PM +0800, Lai Jiangshan wrote:
> 
> It seems that struct trace_seq is too heavy for tracing_stats_read().
> we use simple buffer instead of struct trace_seq.
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 27fecf8..aa1cd92 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -3807,27 +3807,22 @@ tracing_stats_read(struct file *filp, char __user *ubuf,
>  {
>  	unsigned long cpu = (unsigned long)filp->private_data;
>  	struct trace_array *tr = &global_trace;
> -	struct trace_seq *s;
>  	unsigned long cnt;
>  
> -	s = kmalloc(sizeof(*s), GFP_KERNEL);
> -	if (!s)
> -		return -ENOMEM;
> -
> -	trace_seq_init(s);
> +	unsigned char buf[128];



No problem with the conversion. But perhaps keep using a dynamic
buffer as 128 is a bit big for a kernel stack.


> +	int len = 0;
>  
>  	cnt = ring_buffer_entries_cpu(tr->buffer, cpu);
> -	trace_seq_printf(s, "entries: %ld\n", cnt);
> +	len += sprintf(buf + len, "entries: %ld\n", cnt);
>  
>  	cnt = ring_buffer_overrun_cpu(tr->buffer, cpu);
> -	trace_seq_printf(s, "overrun: %ld\n", cnt);
> +	len += sprintf(buf + len, "overrun: %ld\n", cnt);
>  
>  	cnt = ring_buffer_commit_overrun_cpu(tr->buffer, cpu);
> -	trace_seq_printf(s, "commit overrun: %ld\n", cnt);
> -
> -	count = simple_read_from_buffer(ubuf, count, ppos, s->buffer, s->len);
> +	len += sprintf(buf + len, "commit overrun: %ld\n", cnt);
>  
> -	kfree(s);
> +	BUG_ON(len >= sizeof(buf)); /* should not happen, 128 is enough. */
> +	count = simple_read_from_buffer(ubuf, count, ppos, buf, len);
>  
>  	return count;
>  }
> 

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