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Date:   Mon, 20 Nov 2017 21:33:23 +0800
From:   "Du, Changbin" <changbin.du@intel.com>
To:     changbin.du@intel.com
Cc:     rostedt@goodmis.org, mingo@redhat.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] tracing: Allocate mask_str buffer dynamically
Message-ID: <20171120133322.q2sf2duf6kgirqo4@intel.com>
References: <1509506888-4053-1-git-send-email-changbin.du@intel.com>
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Hi Steven,
Have you picked up this patch or need more polish? Thanks.

On Wed, Nov 01, 2017 at 11:28:08AM +0800, changbin.du@intel.com wrote:
> From: Changbin Du <changbin.du@intel.com>
>=20
> 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.
>=20
> 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.
>=20
> The exact buffer size should be:
>   DIV_ROUND_UP(nr_cpu_ids, 4) + nr_cpu_ids/32 + 2;
>=20
> Example output:
>   ff,ffffffff
>=20
> With this change, the mutext tracing_cpumask_update_lock also can be
> removed now, which was used to protect mask_str.
>=20
> Signed-off-by: Changbin Du <changbin.du@intel.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
>=20
> ---
> v3:
>   - remove tracing_cpumask_update_lock which was used to protect mask_str=
=2E (Rostedt)
> v2:
>   - remove 'static' declaration.
>   - fix buffer size.
> ---
>  kernel/trace/trace.c | 29 +++++++++--------------------
>  1 file changed, 9 insertions(+), 20 deletions(-)
>=20
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 752e5da..5d2ec80 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -4178,37 +4178,30 @@ static const struct file_operations show_traces_f=
ops =3D {
>  	.llseek		=3D seq_lseek,
>  };
> =20
> -/*
> - * 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 =3D file_inode(filp)->i_private;
> +	char *mask_str;
>  	int len;
> =20
> -	mutex_lock(&tracing_cpumask_update_lock);
> +	/* Bitmap, ',' and two more bytes for the newline and '\0'. */
> +	len =3D DIV_ROUND_UP(nr_cpu_ids, 4) + nr_cpu_ids/32 + 2;
> +	mask_str =3D kmalloc(len, GFP_KERNEL);
> +	if (!mask_str)
> +		return -ENOMEM;
> =20
> -	len =3D snprintf(mask_str, count, "%*pb\n",
> +	len =3D snprintf(mask_str, len, "%*pb\n",
>  		       cpumask_pr_args(tr->tracing_cpumask));
>  	if (len >=3D count) {
>  		count =3D -EINVAL;
>  		goto out_err;
>  	}
> -	count =3D simple_read_from_buffer(ubuf, count, ppos, mask_str, NR_CPUS+=
1);
> +	count =3D simple_read_from_buffer(ubuf, count, ppos, mask_str, len);
> =20
>  out_err:
> -	mutex_unlock(&tracing_cpumask_update_lock);
> +	kfree(mask_str);
> =20
>  	return count;
>  }
> @@ -4228,8 +4221,6 @@ tracing_cpumask_write(struct file *filp, const char=
 __user *ubuf,
>  	if (err)
>  		goto err_unlock;
> =20
> -	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();
> =20
>  	cpumask_copy(tr->tracing_cpumask, tracing_cpumask_new);
> -
> -	mutex_unlock(&tracing_cpumask_update_lock);
>  	free_cpumask_var(tracing_cpumask_new);
> =20
>  	return count;
> --=20
> 2.7.4
>=20

--=20
Thanks,
Changbin Du

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