linux-kernel - Re: [PATCHv4 05/13] Documentation/ABI: Add new node sysfs attributes

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Date:   Sat, 19 Jan 2019 08:56:02 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Keith Busch <keith.busch@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCHv4 05/13] Documentation/ABI: Add new node sysfs attributes

On Sat, Jan 19, 2019 at 1:01 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Fri, Jan 18, 2019 at 01:08:02PM -0800, Dan Williams wrote:
> > On Thu, Jan 17, 2019 at 3:41 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
> > >
> > > On Wed, Jan 16, 2019 at 6:59 PM Keith Busch <keith.busch@...el.com> wrote:
> > > >
> > > > Add entries for memory initiator and target node class attributes.
> > > >
> > > > Signed-off-by: Keith Busch <keith.busch@...el.com>
> > >
> > > I would recommend combining this with the previous patch, as the way
> > > it is now I need to look at two patches at the time. :-)
> > >
> > > > ---
> > > >  Documentation/ABI/stable/sysfs-devices-node | 25 ++++++++++++++++++++++++-
> > > >  1 file changed, 24 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/Documentation/ABI/stable/sysfs-devices-node b/Documentation/ABI/stable/sysfs-devices-node
> > > > index 3e90e1f3bf0a..a9c47b4b0eee 100644
> > > > --- a/Documentation/ABI/stable/sysfs-devices-node
> > > > +++ b/Documentation/ABI/stable/sysfs-devices-node
> > > > @@ -90,4 +90,27 @@ Date:                December 2009
> > > >  Contact:       Lee Schermerhorn <lee.schermerhorn@...com>
> > > >  Description:
> > > >                 The node's huge page size control/query attributes.
> > > > -               See Documentation/admin-guide/mm/hugetlbpage.rst
> > > > \ No newline at end of file
> > > > +               See Documentation/admin-guide/mm/hugetlbpage.rst
> > > > +
> > > > +What:          /sys/devices/system/node/nodeX/classY/
> > > > +Date:          December 2018
> > > > +Contact:       Keith Busch <keith.busch@...el.com>
> > > > +Description:
> > > > +               The node's relationship to other nodes for access class "Y".
> > > > +
> > > > +What:          /sys/devices/system/node/nodeX/classY/initiator_nodelist
> > > > +Date:          December 2018
> > > > +Contact:       Keith Busch <keith.busch@...el.com>
> > > > +Description:
> > > > +               The node list of memory initiators that have class "Y" access
> > > > +               to this node's memory. CPUs and other memory initiators in
> > > > +               nodes not in the list accessing this node's memory may have
> > > > +               different performance.
> > >
> > > This does not follow the general "one value per file" rule of sysfs (I
> > > know that there are other sysfs files with more than one value in
> > > them, but it is better to follow this rule as long as that makes
> > > sense).
> > >
> > > > +
> > > > +What:          /sys/devices/system/node/nodeX/classY/target_nodelist
> > > > +Date:          December 2018
> > > > +Contact:       Keith Busch <keith.busch@...el.com>
> > > > +Description:
> > > > +               The node list of memory targets that this initiator node has
> > > > +               class "Y" access. Memory accesses from this node to nodes not
> > > > +               in this list may have differet performance.
> > > > --
> > >
> > > Same here.
> > >
> > > And if you follow the recommendation given in the previous message
> > > (add "initiators" and "targets" subdirs under "classX"), you won't
> > > even need the two files above.
> >
> > This recommendation is in conflict with Greg's feedback about kobject
> > usage. If these are just "vanity" subdirs I think it's better to have
> > a multi-value sysfs file. This "list" style is already commonplace for
> > the /sys/devices/system hierarchy.
>
> If you do a subdirectory "correctly" (i.e. a name for an attribute
> group), that's fine.  Just do not ever create a kobject just for a
> subdir, that will mess up userspace.
>
> And I hate the "multi-value" sysfs files, where at all possible, please
> do not copy past bad mistakes there.  If you can make them individual
> files, please do that, it makes it easier to maintain and code the
> kernel for.

I agree in general about multi-value sysfs, but in this case we're
talking about a mask. Masks are a single value. That said I can get on
board with calling what 'cpulist' does a design mistake (human
readable mask), but otherwise switching to one file per item in the
mask is a mess for userspace to consume.