Re: [PATCH v2] tracing: Allocate mask_str buffer dynamically

From: Du, Changbin
Date: Tue Oct 31 2017 - 23:03:42 EST


Hi Rostedt,
On Tue, Oct 31, 2017 at 12:19:58PM -0400, Steven Rostedt wrote:
> On Thu, 26 Oct 2017 00:20:28 +0800
> changbin.du@xxxxxxxxx wrote:
>
> > From: Changbin Du <changbin.du@xxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> >
> > ---
> > 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
>
ok, let me remove it in v3.

> >
> > - 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;
> > }
>

--
Thanks,
Changbin Du

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