linux-kernel - Re: [RFC PATCH] PCI: export MSI mode using attributes, not kobjects

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Date:	Thu, 14 Nov 2013 12:33:11 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Veaceslav Falico <vfalico@...hat.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yinghai Lu <yinghai@...nel.org>,
	Knut Petersen <Knut_Petersen@...nline.de>,
	Ingo Molnar <mingo@...nel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Neil Horman <nhorman@...driver.com>
Subject: Re: [RFC PATCH] PCI: export MSI mode using attributes, not kobjects

On Tue, Nov 5, 2013 at 11:55 AM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> On Sat, Nov 2, 2013 at 9:50 AM, Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
>> On Fri, Nov 01, 2013 at 05:40:02PM -0600, Bjorn Helgaas wrote:
>>> On Tue, Oct 29, 2013 at 3:46 PM, Greg Kroah-Hartman
>>> <gregkh@...uxfoundation.org> wrote:
>>> > From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>> >
>>> > The PCI MSI sysfs code is a mess with kobjects for things that don't
>>> > really need to be kobjects.  This patch creates attributes dynamically
>>> > for the MSI interrupts instead of using kobjects.
>>> >
>>> > Note, this does not delete the existing sysfs MSI code, but puts the
>>> > attributes under a "msi_irqs_2" directory for testing / example.
>>> >
>>> > Also note, this removes a directory from the current MSI interrupt sysfs
>>> > code:
>>> >
>>> > old MSI kobjects:
>>> > pci_device
>>> >    └── msi_irqs
>>> >        └── 40
>>> >            └── mode
>>> >
>>> > new MSI attributes:
>>> > pci_device
>>> >    └── msi_irqs_2
>>> >        └── 40
>>> >
>>> > As there was only one file "mode" with the kobject model, the interrupt
>>> > number is now a file that returns the "mode" of the interrupt (msi vs.
>>> > msix).
>>> >
>>> > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>> > ---
>>> >
>>> > Bjorn, I can make up a patch that rips out the existing kobject code
>>> > here, but I figured this patch would make things easier to follow
>>> > instead of having to dig through the removed logic at the same time.
>>> >
>>> > I'll clean up the error handling path for the create attribute logic as
>>> > well, this was just a proof of concept that this could be done.
>>> >
>>> > Do you think that anyone cares about the current mode files in sysfs to
>>> > move things in this manner?
>>>
>>> I like this a lot better than trying to fix all the holes in the
>>> current kobject code.
>>
>> Great.
>>
>>> I have no idea who, if anybody, cares about the "mode" files.  I
>>> assume there's a way to create the "mode" files with attributes, too?
>>> If so, we could replicate the existing structure with one patch, and
>>> simplify it with a second patch, so it would be easier to revert the
>>> directory change while keeping the fix.
>>
>> No, we can't create a 2-level deep attribute at the moment, only one
>> level, like the patch does.
>>
>> Based on Neil's comments, I think we should be fine with this as-is as
>> no one is messing with these files directly (which implies that we could
>> possibly just remove them entirely to save us the overall pain...)
>
> Hmmm.  https://bugzilla.redhat.com/show_bug.cgi?id=744012 suggests
> that irqbalance might be reading these files.

I looked at the current irqbalance on github [1], and I *think* it
never reads the "mode" files.  It reads the entries in the "msi_irqs"
directory, which you're proposing to change from directories to files,
but I think it only uses the names.

It looks like it should be safe at least for irqbalance to make this a
one-level attribute.  It's possible we'll break somebody's scripts,
but I'm willing to try making this change because it really makes the
refcounting much simpler.

Bjorn

[1] https://github.com/Irqbalance/irqbalance/blob/master/classify.c#L357
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