Message-ID: <CAJZ5v0j+GsHZoifmbO_vX=a8VnFPvwuZr5GRyaVRNuHgHgtcrA@mail.gmail.com>
Date:   Mon, 21 Jan 2019 10:54:33 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.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 Sun, Jan 20, 2019 at 6:34 PM Dan Williams <dan.j.williams@...el.com> wrote:
>
> On Sun, Jan 20, 2019 at 8:20 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Sat, Jan 19, 2019 at 5:56 PM Dan Williams <dan.j.williams@...el.com> wrote:
> > >
> > > 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.
> >
> > Can you please refer to my response to Keith?
>
> Ah, ok I missed the patch4 comments and was reading this one in
> isolation... which also bolsters your comment about squashing these
> two patches together.
>
> > If you have "initiators" and "targets" under "classX" and a list of
> > symlinks in each of them, I don't see any kind of a mess here.
>
> In this instance, I think having symlinks at all is "messy" vs just
> having a mask. Yes, you're right, if we have the proposed symlinks
> from patch4 there is no need for these _nodelist attributes, and those
> symlinks would be better under "initiator" and "target" directories.
> However, I'm arguing going the other way, just have the 2 mask
> attributes and no symlinks. The HMAT erodes the concept of "numa
> nodes" typically being a single digit number space per platform. Given
> increasing numbers of memory target types and initiator devices its
> going to be cumbersome to have userspace walk multiple symlinks vs
> just reading a mask and opening the canonical path for a node
> directly.

The symlinks only need to be walked once, however, and after walking
them (once) the user space program can simply create a mask for
itself.

To me, the question here is how to represent a graph in a filesystem,
and since nodes are naturally represented by directories, it is also
natural to represent connections between them as symlinks.

> This is also part of the rationale for only emitting one "class"
> (initiator / target performance profile) by default. There's an N-to-N
> initiator-target description in the HMAT. When / if we decide emit
> more classes the more work userspace would need to do to convert
> directory structures back into data.

I'm not convinced by this argument, as this conversion is a one-off
exercise.  Ultimately, a tool in user space would need to represent a
graph anyway and how it obtains the data to do that doesn't matter too
much to it.

However, IMO there is value in representing the graph in a filesystem
in such a way that, say, a file manager program (without special
modifications) can be used to walk it by a human.

Let alone the one-value-per-file rule of sysfs that doesn't need to be
violated if symlinks are used.

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