Re: RFC: Memory Tiering Kernel Interfaces (v2)

["ying.huang@intel.com" <ying.huang@intel.com>]

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@linux.ibm.com> wrote:
> > 
> > Wei Xu <weixugc@google.com> writes:
> > 
> > > On Thu, May 12, 2022 at 12:12 AM Aneesh Kumar K V
> > > <aneesh.kumar@linux.ibm.com> wrote:
> > > > 
> > > > On 5/12/22 12:33 PM, ying.huang@intel.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?

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