linux-kernel - Re: [RESEND PATCH v3] tracing: Allocate mask

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Date:   Thu, 30 Nov 2017 11:29:00 +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: [RESEND PATCH v3] tracing: Allocate mask_str buffer dynamically

On Wed, Nov 29, 2017 at 10:12:09PM -0500, Steven Rostedt wrote:
> On Wed, 29 Nov 2017 12:42:45 +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
> 
> Um, what if there's more than 64 CPUs, where I have booted several
> boxes that have more. There's going to be more than 1 comma.
>
The commas are calculated by formula. (DIV_ROUND_UP(nr_cpu_ids, 4))

> > 
> > 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@...el.com>
> > Cc: Steven Rostedt <rostedt@...dmis.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;
> 
> This is broken. Instead do:
> 
> 	len = snprintf(NULL, 0, "%*pb\n",
> 		       cpumask_pr_args(tr->tracing_cpumask)) + 1;
> 
> 	mask_str = kmalloc(len, GFP_KERNEL);
> 	[..]
> 	len = snprintf(mask_str, len, "%*pb\n",
> 		       cpumask_pr_args(tr->tracing_cpumask));
> 
> -- Steve
> 
hmm. I never know that snprintf has such usage. This is much better than
calculating it by a formula.

> > +	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;
> 

-- 
Thanks,
Changbin Du