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Message-ID: <20171031121958.7c0e3ab4@gandalf.local.home>
Date: Tue, 31 Oct 2017 12:19:58 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: changbin.du@...el.com
Cc: mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tracing: Allocate mask_str buffer dynamically
On Thu, 26 Oct 2017 00:20:28 +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
>
> Signed-off-by: Changbin Du <changbin.du@...el.com>
>
> ---
> v2:
> - remove 'static' declaration.
> - fix buffer size.
> ---
> kernel/trace/trace.c | 18 ++++++++++--------
> 1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 752e5da..6b70648 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -4184,31 +4184,33 @@ static const struct file_operations show_traces_fops = {
> */
> static DEFINE_MUTEX(tracing_cpumask_update_lock);
The above mutex was used to protect mask_str.
>
> -/*
> - * 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;
>
> + /* Bitmap, ',' and two more bytes for the newline and '\0'. */
> + len = DIV_ROUND_UP(nr_cpu_ids, 4) + nr_cpu_ids/32 + 2;
> + mask_str = kmalloc(len, GFP_KERNEL);
> + if (!mask_str)
> + return -ENOMEM;
> +
> mutex_lock(&tracing_cpumask_update_lock);
This patch can remove the mutex as well, since there's no sharing of
the mask anymore.
-- Steve
>
> - 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;
> }
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