linux-kernel - Re: [PATCH] specific do_timer

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Date:	Wed, 30 Nov 2011 20:34:23 -0600
From:	Dimitri Sivanich <sivanich@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] specific do_timer_cpu value for nohz off mode

On Wed, Nov 30, 2011 at 06:12:05PM -0800, Andrew Morton wrote:
> On Wed, 30 Nov 2011 20:06:31 -0600 Dimitri Sivanich <sivanich@....com> wrote:
> 
> > On Wed, Nov 30, 2011 at 04:11:31PM -0800, Andrew Morton wrote:
> > > On Wed, 30 Nov 2011 09:29:59 -0600
> > > Dimitri Sivanich <sivanich@....com> wrote:
> > > 
> > > > +static ssize_t sysfs_store_do_timer_cpu(struct sys_device *dev,
> > > > +						struct sysdev_attribute *attr,
> > > > +						const char *buf, size_t size)
> > > > +{
> > > > +	struct sysdev_ext_attribute *ea = SYSDEV_TO_EXT_ATTR(attr);
> > > > +	unsigned int new;
> > > > +	int rv;
> > > > +
> > > > +#ifdef CONFIG_NO_HZ
> > > > +	/* nohz mode not supported */
> > > > +	if (tick_nohz_enabled)
> > > > +		return -EINVAL;
> > > > +#endif
> > > > +
> > > > +	rv = kstrtouint(buf, 0, &new);
> > > > +	if (rv)
> > > > +		return rv;
> > > > +
> > > > +	if (new >= NR_CPUS || !cpu_online(new))
> > > > +		return -ERANGE;
> > > > +
> > > > +	*(unsigned int *)(ea->var) = new;
> > > > +	return size;
> > > > +}
> > > 
> > > checkpatch tells us:
> > > 
> > > WARNING: usage of NR_CPUS is often wrong - consider using cpu_possible(), num_possible_cpus(), for_each_possible_cpu(), etc
> > 
> > I think a check against num_possible_cpus() should be OK.
> 
> You want cpu_possible().  Or maybe cpu_present().

May be splitting hairs, but I think I like num_present_cpus even better.

> 
> > > 
> > > I think the check can just be removed?  Surely cpu_online(1000000000)
> > > will return false?
> > 
> > A value > NR_CPUS and < MAX_INT caused a panic in sysfs_store_do_timer_cpu,
> > presumably from the cpu_online() check.  The check against NR_CPUS avoided
> > the panic.
> 
> OK.  Well, it's not a panic:

Actually, I didn't have CONFIG_DEBUG_PER_CPU_MAPS turned on.

> 
> static inline unsigned int cpumask_check(unsigned int cpu)
> {
> #ifdef CONFIG_DEBUG_PER_CPU_MAPS
> 	WARN_ON_ONCE(cpu >= nr_cpumask_bits);
> #endif /* CONFIG_DEBUG_PER_CPU_MAPS */
> 	return cpu;
> }
> 
> so we can't do cpu_online(insane number)
> 
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