Re: RFC: Memory Tiering Kernel Interfaces (v2)

[Wei Xu <weixugc@google.com>]

On Thu, May 26, 2022 at 2:32 AM Jonathan Cameron
<Jonathan.Cameron@huawei.com> wrote:
>
> On Wed, 25 May 2022 10:27:42 -0700
> Wei Xu <weixugc@google.com> wrote:
>
> > On Wed, May 25, 2022 at 3:01 AM Aneesh Kumar K V
> > <aneesh.kumar@linux.ibm.com> wrote:
> > >
> > > On 5/25/22 2:33 PM, Ying Huang wrote:
> > > > On Tue, 2022-05-24 at 22:32 -0700, Wei Xu wrote:
> > > >> On Tue, May 24, 2022 at 1:24 AM Ying Huang <ying.huang@intel.com> wrote:
> > > >>>
> > > >>> On Tue, 2022-05-24 at 00:04 -0700, Wei Xu wrote:
> > > >>>> On Thu, May 19, 2022 at 8:06 PM Ying Huang <ying.huang@intel.com> wrote:
> > > >>>>>
> > >
> > > ...
> > >
> > > >
> > > > OK.  Just to confirm.  Does this mean that we will have fixed device ID,
> > > > for example,
> > > >
> > > > GPU                   memtier255
> > > > DRAM (with CPU)               memtier0
> > > > PMEM                  memtier1
> > > >
> > > > When we add a new memtier, it can be memtier254, or memter2?  The rank
> > > > value will determine the real demotion order.
> > > >
> > > > I think you may need to send v3 to make sure everyone is at the same
> > > > page.
> > > >
> > >
> > > What we have implemented which we will send as RFC shortly is below.
> > >
> > > cd /sys/dekvaneesh@ubuntu-guest:~$ cd /sys/devices/system/
> > > kvaneesh@ubuntu-guest:/sys/devices/system$ pwd
> > > /sys/devices/system
> > > kvaneesh@ubuntu-guest:/sys/devices/system$ ls
> > > clockevents  clocksource  container  cpu  edac  memory  memtier  mpic
> > > node  power
> > > kvaneesh@ubuntu-guest:/sys/devices/system$ cd memtier/
> > > kvaneesh@ubuntu-guest:/sys/devices/system/memtier$ pwd
> > > /sys/devices/system/memtier
> > > kvaneesh@ubuntu-guest:/sys/devices/system/memtier$ ls
> > > default_rank  max_rank  memtier1  power  uevent
> > > kvaneesh@ubuntu-guest:/sys/devices/system/memtier$ cat default_rank
> > > 1
> > > kvaneesh@ubuntu-guest:/sys/devices/system/memtier$ cat max_rank
> > > 3
> >
> > For flexibility, we don't want max_rank to be interpreted as the
> > number of memory tiers.  Also, we want to leave spaces in rank values
> > to allow new memtiers to be inserted when needed.  So I'd suggest to
> > make max_rank a much larger value (e.g. 255).
> >
> > > kvaneesh@ubuntu-guest:/sys/devices/system/memtier$ cd memtier1/
> > > kvaneesh@ubuntu-guest:/sys/devices/system/memtier/memtier1$ ls
> > > nodelist  power  rank  subsystem  uevent
> > > kvaneesh@ubuntu-guest:/sys/devices/system/memtier/memtier1$ cat nodelist
> > > 0-3
> > > kvaneesh@ubuntu-guest:/sys/devices/system/memtier/memtier1$ cat rank
> > > 1
> > > kvaneesh@ubuntu-guest:/sys/devices/system/memtier/memtier1$ cd
> > > ../../node/node1/
> > > kvaneesh@ubuntu-guest:/sys/devices/system/node/node1$ cat memtier
> > > 1
> > > kvaneesh@ubuntu-guest:/sys/devices/system/node/node1$
> > > root@ubuntu-guest:/sys/devices/system/node/node1# echo 0 > memtier
> > > root@ubuntu-guest:/sys/devices/system/node/node1# cat memtier
> > > 0
> > > root@ubuntu-guest:/sys/devices/system/node/node1# cd ../../memtier/
> > > root@ubuntu-guest:/sys/devices/system/memtier# ls
> > > default_rank  max_rank  memtier0  memtier1  power  uevent
> > > root@ubuntu-guest:/sys/devices/system/memtier# cd memtier0/
> > > root@ubuntu-guest:/sys/devices/system/memtier/memtier0# cat nodelist
> > > 1
> > > root@ubuntu-guest:/sys/devices/system/memtier/memtier0# cat rank
> > > 0
> >
> > It looks like the example here demonstrates the dynamic creation of
> > memtier0.  If so, how is the rank of memtier0 determined?  If we want
> > to support creating new memtiers at runtime, I think an explicit
> > interface that specifies both device ID and rank is preferred to avoid
> > implicit dependencies between device IDs and ranks.
>
> Why make device ID explicit - it's meaningless I think?
> How about a creation interface that is simply writing the rank value
> to create a new one?  The only race I can see would be to get
> two parallel attempts to create a new tier with the same rank.
> That seems unlikely to matter unless we support changing rank later.
>
> Two attempts to create the same device ID tier seems more likely to
> cause fiddly races.

That's right: Device ID is not needed when creating a new memtier. It
should be enough to provide only a rank value.

> Jonathan
>
>
> >
> > > root@ubuntu-guest:/sys/devices/system/memtier/memtier0# echo 4 > rank
> > > bash: rank: Permission denied
> > > root@ubuntu-guest:/sys/devices/system/memtier/memtier0#
>
>