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SubjectRe: [PATCH v4] tracing: Allocate mask_str buffer dynamically

Hi Rostedt, How about this version?

On Thu, Nov 30, 2017 at 11:39:43AM +0800, changbin.du@intel.com wrote:
> From: Changbin Du <changbin.du@intel.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.
>
> 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@intel.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
>
> ---
> v4:
> - calculate the buffer size using snprintf. (Rostedt)
> 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..53d29c7 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);
> + len = snprintf(NULL, 0, "%*pb\n",
> + cpumask_pr_args(tr->tracing_cpumask)) + 1;
> + 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;
> --
> 2.7.4
>

--
Thanks,
Changbin Du

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