linux-kernel - Re: [PATCH RFC: linux-next 1/2] irq: Add CPU mask affinity hint callback framework

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Message-ID: <alpine.LFD.2.00.1004271809080.2951@localhost.localdomain>
Date:	Wed, 28 Apr 2010 18:45:21 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@...el.com>
cc:	"davem@...emloft.net" <davem@...emloft.net>,
	"arjan@...ux.jf.intel.com" <arjan@...ux.jf.intel.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC: linux-next 1/2] irq: Add CPU mask affinity hint
 callback framework

B1;2005;0cPeter,

On Tue, 27 Apr 2010, Peter P Waskiewicz Jr wrote:
> On Tue, 27 Apr 2010, Thomas Gleixner wrote:
> > On Sun, 18 Apr 2010, Peter P Waskiewicz Jr wrote:
> > > +/**
> > > + * struct irqaffinityhint - per interrupt affinity helper
> > > + * @callback:	device driver callback function
> > > + * @dev:	reference for the affected device
> > > + * @irq:	interrupt number
> > > + */
> > > +struct irqaffinityhint {
> > > +	irq_affinity_hint_t callback;
> > > +	void *dev;
> > > +	int irq;
> > > +};
> > 
> > Why do you need that extra data structure ? The device and the irq
> > number are known, so all you need is the callback itself. So no need
> > for allocating memory ....
> 
> When I register the function callback with the interrupt layer, I need to
> know what device structures to reference back in the driver.  In other words,
> if I call into an underlying driver with just an interrupt number, then I
> have no way at getting at the dev structures (netdevice for me, plus my
> private adapter structures), unless I declare them globally (yuck).

Grr, I knew that I missed something. That'll teach me to review
patches before the coffee has reached my brain cells :)

> I had a different approach before this one where I assumed the device from
> the irq handler callback was safe to use for the device in this new callback.
> I didn't feel really great about that, since it's an implicit assumption that
> could cause things to go sideways really quickly.
>
> Let me know what you think either way.  I'm certainly willing to make a
> change, I just don't know at this point what's the safest approach from what
> I currently have.

So you need a reference to your device, so what about the following:

struct irq_affinity_hint;

struct irq_affinity_hint {
       unsigned int (*callback)(unsigned int irq, struct irq_affinity_hint *hint,
				cpumask_var_t *mask);
}

Now you embed that struct into your device private data structure and
you get the reference to it back in the callback function. No extra
kmalloc/kfree, less code.

One other thing I noticed, but forgot to comment on:

> +static int irq_affinity_hint_proc_show(struct seq_file *m, void *v)
> +{
> +	struct irq_desc *desc = irq_to_desc((long)m->private);
> +	struct cpumask mask;
> +	unsigned int ret = 0;

 Why do we return 0, when there is no callback and no hint available ?

> +

  We don't want to have cpumask enforced on stack. Please make that:

     	cpumask_var_t mask;

	if (!alloc_cpumask_var(&mask, GFP_KERNEL))
	       return -ENOMEM;

> +	if (desc->hint && desc->hint->callback) {

  The access to desc-> needs to be protected with
  desc->lock. Otherwise you might race with a callback unregister.

> +		ret = desc->hint->callback(&mask, (long)m->private,
> +		                           desc->hint->dev);
> +		if (!ret)
> +			seq_cpumask(m, &mask);
> +	}
> +
> +	seq_putc(m, '\n');
> +	return ret;
> +}

Thanks,

	tglx
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