netdev - Re: [PATCH linux-next v2 1/2] irq: Add CPU mask affinity hint

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Message-ID: <Pine.WNT.4.64.1004301212060.6636@PPWASKIE-MOBL2.amr.corp.intel.com>
Date:	Fri, 30 Apr 2010 12:13:01 -0700 (Pacific Daylight Time)
From:	Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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 linux-next v2 1/2] irq: Add CPU mask affinity hint

On Fri, 30 Apr 2010, Thomas Gleixner wrote:

> On Fri, 30 Apr 2010, Peter P Waskiewicz Jr wrote:
>> On Fri, 30 Apr 2010, Thomas Gleixner wrote:
>>>> +extern int irq_register_affinity_hint(unsigned int irq,
>>>> +                                      const struct cpumask *m);
>>>
>>> I think we can do with a single funtion irq_set_affinity_hint() and
>>> let the caller set the pointer to NULL.
>>
>> Ok, I've been running into some issues.  If CONFIG_CPUMASK_OFFSTACK is not
>> set, then cpumask_var_t structs are single-element arrays that cannot be
>> NULL'd out.  I'm pretty sure I need to keep the unregister part of the API.
>> Thoughts?
>
> extern int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m);
>
> So why should calling irq_set_affinity_hint(irqnr, NULL) not work ?

What was that you said about coffee and brain cells?  :-)

>
>> I just looked at the original show_affinity function, and it does not grab
>> desc->lock before copying mask out of desc.  Should I follow that model, or
>> should I fix that function to honor desc->lock?
>
> desc->affinity can only race against something changing the affinity
> bits, so that just might return some random data.
>
> In the hint case the irq could be shut down and the affinity hint
> could be freed while you are accessing it. Not a good idea :)

Good point.

Latest spin coming shortly.  Thanks for the quick feedback!

-PJ
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