netdev - Re: [PATCH mlx5-next v7 0/4] Dynamically assign MSI-X vectors count

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Date:   Wed, 31 Mar 2021 14:07:20 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Keith Busch <kbusch@...nel.org>,
        Leon Romanovsky <leon@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Jakub Kicinski <kuba@...nel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        linux-rdma@...r.kernel.org, Netdev <netdev@...r.kernel.org>,
        Don Dutile <ddutile@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH mlx5-next v7 0/4] Dynamically assign MSI-X vectors count

On Wed, Mar 31, 2021 at 05:03:45PM +0200, Greg Kroah-Hartman wrote:
> > It isn't a struct device object at all though, it just organizing
> > attributes.
> 
> That's the point, it really is not.  You are forced to create a real
> object for that subdirectory, and by doing so, you are "breaking" the
> driver/device model.  As is evident by userspace not knowing what is
> going on here.

I'm still not really sure about what this means in practice..

I found an nested attribute in RDMA land so lets see how it behaves.
 
   /sys/class/infiniband/ibp0s9/ <-- This is a struct device/ib_device

Then we have 261 'attribute' files under a ports subdirectory, for
instance:

 /sys/class/infiniband/ibp0s9/ports/1/cm_tx_retries/dreq

Open/read works fine, and the specialty userspace that people built on
this has been working for a long time.

Does udev see the deeply nested attributes? Apparently yes:

$ udevadm info -a /sys/class/infiniband/ibp0s9
    ATTR{ports/1/cm_rx_duplicates/dreq}=="0"
    [..]

Given your remarks, I'm surprised, but it seems to work - I assume if
udevadm shows it then all the rules will work too.

Has udev become confused about what is a struct device? Looks like no:

$ udevadm info -a /sys/class/infiniband/ibp0s9/port
Unknown device "/sys/class/infiniband/ibp0s9/port": No such device

Can you give an example where things go wrong?

(and I inherited this RDMA stuff. In the last two years we moved it
 all to netlink and modern userspace largely no longer touches sysfs,
 but I can't break in-use uAPI)

> > > Does that help?  The rules are:
> > > 	- Once you use a 'struct device', all subdirs below that device
> > > 	  are either an attribute group for that device or a child
> > > 	  device.
> > > 	- A struct device can NOT have an attribute group as a parent,
> > > 	  it can ONLY have another struct device as a parent.
> > > 
> > > If you break those rules, the kernel has the ability to get really
> > > confused unless you are very careful, and userspace will be totally lost
> > > as you can not do anything special there.
> > 
> > The kernel gets confused?
> 
> Putting a kobject as a child of a struct device can easily cause
> confusion as that is NOT what you should be doing.  Especially if you
> then try to add a device to be a child of that kobject. 

That I've never seen. I've only seen people making extra levels of
directories for organizing attributes.

> > How do you fix them? It is uAPI at this point so we can't change the
> > directory names. Can't make them struct devices (userspace would get
> > confused if we add *more* sysfs files)
> 
> How would userspace get confused?  If anything it would suddenly "wake
> up" and see these attributes properly.

We are talking about specialty userspace that is designed to work with
the sysfs layout as-is. Not udev. In some of these subdirs the
userspace does readdir() on - if you start adding random stuff it will
break it.

> > Since it seems like kind of a big problem can we make this allowed
> > somehow?
> 
> No, not at all.  Please do not do that.  I will look into the existing
> users and try to see if I can fix them up.  Maybe start annoying people
> by throwing warnings if you try to register a kobject as a child of a
> device...

How does that mesh with our don't break userspace ideal?? :(

> > Well, from what I understand, it wont be used because udev can't do
> > three level deep attributes, and if that hasn't been a problem in that
> > last 10 years for the existing places, it might not ever be needed in
> > udev at all.
> 
> If userspace is not seeing these attributes then WHY CREATE THEM AT
> ALL???

*udev* is not the userspace! People expose sysfs attributes and then
make specialty userspace to consume them! I've seen it many times now.

> Seriously, what is needing to see these in sysfs if not the tools that
> we have today to use sysfs?  Are you wanting to create new tools instead
> to handle these new attributes?  Maybe just do not create them in the
> first place?

This advice is about 10 years too late :(

Regardless, lets not do deeply nested attributes here in PCI. They are
PITA anyhow.

Jason