linux-kernel - Re: [PATCH v2] tracing: Allocate mask

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Message-ID: <20171101025615.GA29285@intel.com>
Date:   Wed, 1 Nov 2017 10:56:15 +0800
From:   "Du, Changbin" <changbin.du@...el.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     changbin.du@...el.com, mingo@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tracing: Allocate mask_str buffer dynamically

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@...el.com wrote:
> 
> > From: Changbin Du <changbin.du@...el.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
> > 
> > Signed-off-by: Changbin Du <changbin.du@...el.com>
> > 
> > ---
> > 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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