tracing: Allocate mask_str buffer dynamically

commit 90e406f96f630c07d631a021fd4af10aac913e77 upstream.

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.

Link: http://lkml.kernel.org/r/1512013183-19107-1-git-send-email-changbin.du@intel.com
Fixes: 36dfe9252bd4c ("ftrace: make use of tracing_cpumask")
Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c1e50cc0d7b009ea810b65ac54ba0b094cbc9942..4214cd960b8ed432c1de3bbdac2de65cbe6917e7 100644 (file)
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3727,37 +3727,30 @@ 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;
 }
@@ -3777,8 +3770,6 @@ 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) {
@@ -3801,8 +3792,6 @@ 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;