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SubjectRe: RFC: Memory Tiering Kernel Interfaces (v2)
On Thu, May 12, 2022 at 7:53 PM ying.huang@intel.com
<ying.huang@intel.com> wrote:
>
> On Thu, 2022-05-12 at 16:37 +0800, ying.huang@intel.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@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?
>
> My words may be confusing, so let me say it in another way.

I can understand. :)

> 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.

"demote" might be fine to add there. Just to clarify, we (Google)
currently don't yet have the need for an interface to do system-wide
demotion from one tier to another. What we need is memory.demote
(similar to memory.reclaim) for memory cgroup based demotions.

Other things that might be added include tier-specific properties
(e.g. expected latency and bandwidth when available) and tier-specific
stats.

Under /sys/devices/system/memtier/, we may add global properties about
memory tiers, e.g. max number of tiers, min/max tier ids (which might
be useful if we hide unpopulated memory tiers).

> Or, is it necessary to add
>
> /sys/devices/system/memtier/memtierN/meminfo
> /sys/devices/system/memtier/memtierN/vmstat

The userspace can aggregate such data from node/nodeN/{meminfo,
vmstat} based on the memory tier nodelist. But I am not against adding
these files to memtierN/ for user convenience.

> 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.

This sounds good. I think a memtier directory may eventually become a
necessity, though I don't feel too strongly about adding it right now.

> 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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