Date:   Fri, 13 May 2022 10:52:58 +0800
From:   "ying.huang@...el.com" <ying.huang@...el.com>
To:     Wei Xu <weixugc@...gle.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Greg Thelen <gthelen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Michal Hocko <mhocko@...nel.org>,
        Tim C Chen <tim.c.chen@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Alistair Popple <apopple@...dia.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Feng Tang <feng.tang@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Dan Williams <dan.j.williams@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        Linux MM <linux-mm@...ck.org>,
        Brice Goglin <brice.goglin@...il.com>,
        Hesham Almatary <hesham.almatary@...wei.com>
Subject: Re: RFC: Memory Tiering Kernel Interfaces (v2)

On Thu, 2022-05-12 at 16:37 +0800, ying.huang@...el.com wrote:
> On Thu, 2022-05-12 at 01:15 -0700, Wei Xu wrote:
> > On Thu, May 12, 2022 at 12:36 AM Aneesh Kumar K.V
> > <aneesh.kumar@...ux.ibm.com> wrote:
> > > 
> > > Wei Xu <weixugc@...gle.com> writes:
> > > 
> > > > On Thu, May 12, 2022 at 12:12 AM Aneesh Kumar K V
> > > > <aneesh.kumar@...ux.ibm.com> wrote:
> > > > > 
> > > > > On 5/12/22 12:33 PM, ying.huang@...el.com wrote:
> > > > > > On Wed, 2022-05-11 at 23:22 -0700, Wei Xu wrote:
> > > > > > > Sysfs Interfaces
> > > > > > > ================
> > > > > > > 
> > > > > > > * /sys/devices/system/memtier/memtierN/nodelist
> > > > > > > 
> > > > > > >    where N = 0, 1, 2 (the kernel supports only 3 tiers for now).
> > > > > > > 
> > > > > > >    Format: node_list
> > > > > > > 
> > > > > > >    Read-only.  When read, list the memory nodes in the specified tier.
> > > > > > > 
> > > > > > >    Tier 0 is the highest tier, while tier 2 is the lowest tier.
> > > > > > > 
> > > > > > >    The absolute value of a tier id number has no specific meaning.
> > > > > > >    What matters is the relative order of the tier id numbers.
> > > > > > > 
> > > > > > >    When a memory tier has no nodes, the kernel can hide its memtier
> > > > > > >    sysfs files.
> > > > > > > 
> > > > > > > * /sys/devices/system/node/nodeN/memtier
> > > > > > > 
> > > > > > >    where N = 0, 1, ...
> > > > > > > 
> > > > > > >    Format: int or empty
> > > > > > > 
> > > > > > >    When read, list the memory tier that the node belongs to.  Its value
> > > > > > >    is empty for a CPU-only NUMA node.
> > > > > > > 
> > > > > > >    When written, the kernel moves the node into the specified memory
> > > > > > >    tier if the move is allowed.  The tier assignment of all other nodes
> > > > > > >    are not affected.
> > > > > > > 
> > > > > > >    Initially, we can make this interface read-only.
> > > > > > 
> > > > > > It seems that "/sys/devices/system/node/nodeN/memtier" has all
> > > > > > information we needed.  Do we really need
> > > > > > "/sys/devices/system/memtier/memtierN/nodelist"?
> > > > > > 
> > > > > > That can be gotten via a simple shell command line,
> > > > > > 
> > > > > > $ grep . /sys/devices/system/node/nodeN/memtier | sort -n -k 2 -t ':'
> > > > > > 
> > > > > 
> > > > > It will be really useful to fetch the memory tier node list in an easy
> > > > > fashion rather than reading multiple sysfs directories. If we don't have
> > > > > other attributes for memorytier, we could keep
> > > > > "/sys/devices/system/memtier/memtierN" a NUMA node list there by
> > > > > avoiding /sys/devices/system/memtier/memtierN/nodelist
> > > > > 
> > > > > -aneesh
> > > > 
> > > > It is harder to implement memtierN as just a file and doesn't follow
> > > > the existing sysfs pattern, either.  Besides, it is extensible to have
> > > > memtierN as a directory.
> > > 
> > > diff --git a/drivers/base/node.c b/drivers/base/node.c
> > > index 6248326f944d..251f38ec3816 100644
> > > --- a/drivers/base/node.c
> > > +++ b/drivers/base/node.c
> > > @@ -1097,12 +1097,49 @@ static struct attribute *node_state_attrs[] = {
> > >         NULL
> > >  };
> > > 
> > > +#define MAX_TIER 3
> > > +nodemask_t memory_tier[MAX_TIER];
> > > +
> > > +#define _TIER_ATTR_RO(name, tier_index)                                        \
> > > +       { __ATTR(name, 0444, show_tier, NULL), tier_index, NULL }
> > > +
> > > +struct memory_tier_attr {
> > > +       struct device_attribute attr;
> > > +       int tier_index;
> > > +       int (*write)(nodemask_t nodes);
> > > +};
> > > +
> > > +static ssize_t show_tier(struct device *dev,
> > > +                        struct device_attribute *attr, char *buf)
> > > +{
> > > +       struct memory_tier_attr *mt = container_of(attr, struct memory_tier_attr, attr);
> > > +
> > > +       return sysfs_emit(buf, "%*pbl\n",
> > > +                         nodemask_pr_args(&memory_tier[mt->tier_index]));
> > > +}
> > > +
> > >  static const struct attribute_group memory_root_attr_group = {
> > >         .attrs = node_state_attrs,
> > >  };
> > > 
> > > +
> > > +#define TOP_TIER 0
> > > +static struct memory_tier_attr memory_tiers[] = {
> > > +       [0] = _TIER_ATTR_RO(memory_top_tier, TOP_TIER),
> > > +};
> > > +
> > > +static struct attribute *memory_tier_attrs[] = {
> > > +       &memory_tiers[0].attr.attr,
> > > +       NULL
> > > +};
> > > +
> > > +static const struct attribute_group memory_tier_attr_group = {
> > > +       .attrs = memory_tier_attrs,
> > > +};
> > > +
> > >  static const struct attribute_group *cpu_root_attr_groups[] = {
> > >         &memory_root_attr_group,
> > > +       &memory_tier_attr_group,
> > >         NULL,
> > >  };
> > > 
> > > 
> > > As long as we have the ability to see the nodelist, I am good with the
> > > proposal.
> > > 
> > > -aneesh
> > 
> > I am OK with moving back the memory tier nodelist into node/.  When
> > there are more memory tier attributes needed, we can then create the
> > memory tier subtree and replace the tier nodelist in node/ with
> > symlinks.
> 
> What attributes do you imagine that we may put in memory_tierX/ sysfs
> directory?  If we have good candidates in mind, we may just do that. 
> What I can imagine now is "demote", like "memory_reclaim" in nodeX/ or
> node/ directory you proposed before.  Is it necessary to show something
> like "meminfo", "vmstat" there?

My words may be confusing, so let me say it in another way.

Just for brainstorm, if we have

  /sys/devices/system/memtier/memtierN/

What can we put in it in addition to "nodelist" or links to the nodes? 
For example,

  /sys/devices/system/memtier/memtierN/demote

When write a page number to it, the specified number of pages will be
demoted from memtierN to memtierN+1, like the
/sys/devices/system/node/memory_reclaim interface you proposed before. 
Or, is it necessary to add

  /sys/devices/system/memtier/memtierN/meminfo
  /sys/devices/system/memtier/memtierN/vmstat

I don't mean to propose these.  Just want to know whether there's
requirement for these kind of stuff?  And what else may be required.

Best Regards,
Huang, Ying

> > 
> > So the revised sysfs interfaces are:
> > 
> > * /sys/devices/system/node/memory_tierN (read-only)
> > 
> >   where N = 0, 1, 2
> > 
> >   Format: node_list
> > 
> > * /sys/devices/system/node/nodeN/memory_tier (read/write)
> > 
> >   where N = 0, 1, ...
> > 
> >   Format: int or empty
> 

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