On 6/6/22 8:29 PM, Jonathan Cameron wrote:
On Fri, 3 Jun 2022 14:10:47 +0530
Aneesh Kumar K V <aneesh.kumar@xxxxxxxxxxxxx> wrote:
On 5/27/22 7:45 PM, Jonathan Cameron wrote:
On Fri, 27 May 2022 17:55:23 +0530
"Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx> wrote:
From: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>
Add support to read/write the memory tierindex for a NUMA node.
/sys/devices/system/node/nodeN/memtier
where N = node id
When read, It list the memory tier that the node belongs to.
When written, the kernel moves the node into the specified
memory tier, the tier assignment of all other nodes are not
affected.
If the memory tier does not exist, writing to the above file
create the tier and assign the NUMA node to that tier.
creates
There was some discussion in v2 of Wei Xu's RFC that what matter
for creation is the rank, not the tier number.
My suggestion is move to an explicit creation file such as
memtier/create_tier_from_rank
to which writing the rank gives results in a new tier
with the next device ID and requested rank.
I think the below workflow is much simpler.
:/sys/devices/system# cat memtier/memtier1/nodelist
1-3
:/sys/devices/system# cat node/node1/memtier
1
:/sys/devices/system# ls memtier/memtier*
nodelist power rank subsystem uevent
/sys/devices/system# ls memtier/
default_rank max_tier memtier1 power uevent
:/sys/devices/system# echo 2 > node/node1/memtier
:/sys/devices/system#
:/sys/devices/system# ls memtier/
default_rank max_tier memtier1 memtier2 power uevent
:/sys/devices/system# cat memtier/memtier1/nodelist
2-3
:/sys/devices/system# cat memtier/memtier2/nodelist
1
:/sys/devices/system#
ie, to create a tier we just write the tier id/tier index to
node/nodeN/memtier file. That will create a new memory tier if needed
and add the node to that specific memory tier. Since for now we are
having 1:1 mapping between tier index to rank value, we can derive the
rank value from the memory tier index.
For dynamic memory tier support, we can assign a rank value such that
new memory tiers are always created such that it comes last in the
demotion order.
I'm not keen on having to pass through an intermediate state where
the rank may well be wrong, but I guess it's not that harmful even
if it feels wrong ;)
Any new memory tier added can be of lowest rank (rank - 0) and hence
will appear as the highest memory tier in demotion order. User can then
assign the right rank value to the memory tier? Also the actual demotion
target paths are built during memory block online which in most case
would happen after we properly verify that the device got assigned to
the right memory tier with correct rank value?
Races are potentially a bit of a pain though depending on what we
expect the usage model to be.
There are patterns (CXL regions for example) of guaranteeing the
'right' device is created by doing something like
cat create_tier > temp.txt
#(temp gets 2 for example on first call then
# next read of this file gets 3 etc)
cat temp.txt > create_tier
# will fail if there hasn't been a read of the same value
Assuming all software keeps to the model, then there are no
race conditions over creation. Otherwise we have two new
devices turn up very close to each other and userspace scripting
tries to create two new tiers - if it races they may end up in
the same tier when that wasn't the intent. Then code to set
the rank also races and we get two potentially very different
memories in a tier with a randomly selected rank.
Fun and games... And a fine illustration why sysfs based 'device'
creation is tricky to get right (and lots of cases in the kernel
don't).
I would expect userspace to be careful and verify the memory tier and
rank value before we online the memory blocks backed by the device. Even
if we race, the result would be two device not intended to be part of
the same memory tier appearing at the same tier. But then we won't be
building demotion targets yet. So userspace could verify this, move the
nodes out of the memory tier. Once it is verified, memory blocks can be
onlined.
Having said that can you outline the usage of
memtier/create_tier_from_rank ?
-aneesh
