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Subject[RESEND PATCH v3] tracing: Allocate mask_str buffer dynamically
Date
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.

The exact buffer size should be:
DIV_ROUND_UP(nr_cpu_ids, 4) + nr_cpu_ids/32 + 2;

Example output:
ff,ffffffff

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>

---
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;
+ 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
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