Re: [PATCH v2 0/5] mm: demotion: Introduce new node state N_DEMOTION_TARGETS

[Yang Shi <shy828301@gmail.com>]

On Wed, Apr 20, 2022 at 10:41 PM Wei Xu <weixugc@google.com> wrote:
>
> On Wed, Apr 20, 2022 at 8:12 PM Yang Shi <shy828301@gmail.com> wrote:
> >
> > On Thu, Apr 14, 2022 at 12:00 AM ying.huang@intel.com
> > <ying.huang@intel.com> wrote:
> > >
> > > On Wed, 2022-04-13 at 14:52 +0530, Jagdish Gediya wrote:
> > > > Current implementation to find the demotion targets works
> > > > based on node state N_MEMORY, however some systems may have
> > > > dram only memory numa node which are N_MEMORY but not the
> > > > right choices as demotion targets.
> > > >
> > > > This patch series introduces the new node state
> > > > N_DEMOTION_TARGETS, which is used to distinguish the nodes which
> > > > can be used as demotion targets, node_states[N_DEMOTION_TARGETS]
> > > > is used to hold the list of nodes which can be used as demotion
> > > > targets, support is also added to set the demotion target
> > > > list from user space so that default behavior can be overridden.
> > >
> > > It appears that your proposed user space interface cannot solve all
> > > problems.  For example, for system as follows,
> > >
> > > Node 0 & 2 are cpu + dram nodes and node 1 are slow memory node near
> > > node 0,
> > >
> > > available: 3 nodes (0-2)
> > > node 0 cpus: 0 1
> > > node 0 size: n MB
> > > node 0 free: n MB
> > > node 1 cpus:
> > > node 1 size: n MB
> > > node 1 free: n MB
> > > node 2 cpus: 2 3
> > > node 2 size: n MB
> > > node 2 free: n MB
> > > node distances:
> > > node   0   1   2
> > >   0:  10  40  20
> > >   1:  40  10  80
> > >   2:  20  80  10
> > >
> > > Demotion order 1:
> > >
> > > node    demotion_target
> > >  0              1
> > >  1              X
> > >  2              X
> > >
> > > Demotion order 2:
> > >
> > > node    demotion_target
> > >  0              1
> > >  1              X
> > >  2              1
> > >
> > > The demotion order 1 is preferred if we want to reduce cross-socket
> > > traffic.  While the demotion order 2 is preferred if we want to take
> > > full advantage of the slow memory node.  We can take any choice as
> > > automatic-generated order, while make the other choice possible via user
> > > space overridden.
> > >
> > > I don't know how to implement this via your proposed user space
> > > interface.  How about the following user space interface?
> > >
> > > 1. Add a file "demotion_order_override" in
> > >         /sys/devices/system/node/
> > >
> > > 2. When read, "1" is output if the demotion order of the system has been
> > > overridden; "0" is output if not.
> > >
> > > 3. When write "1", the demotion order of the system will become the
> > > overridden mode.  When write "0", the demotion order of the system will
> > > become the automatic mode and the demotion order will be re-generated.
> > >
> > > 4. Add a file "demotion_targets" for each node in
> > >         /sys/devices/system/node/nodeX/
> > >
> > > 5. When read, the demotion targets of nodeX will be output.
> > >
> > > 6. When write a node list to the file, the demotion targets of nodeX
> > > will be set to the written nodes.  And the demotion order of the system
> > > will become the overridden mode.
> >
> > TBH I don't think having override demotion targets in userspace is
> > quite useful in real life for now (it might become useful in the
> > future, I can't tell). Imagine you manage hundred thousands of
> > machines, which may come from different vendors, have different
> > generations of hardware, have different versions of firmware, it would
> > be a nightmare for the users to configure the demotion targets
> > properly. So it would be great to have the kernel properly configure
> > it *without* intervening from the users.
> >
> > So we should pick up a proper default policy and stick with that
> > policy unless it doesn't work well for the most workloads. I do
> > understand it is hard to make everyone happy. My proposal is having
> > every node in the fast tier has a demotion target (at least one) if
> > the slow tier exists sounds like a reasonable default policy. I think
> > this is also the current implementation.
> >
>
> This is reasonable.  I agree that with a decent default policy, the
> overriding of per-node demotion targets can be deferred.  The most
> important problem here is that we should allow the configurations
> where memory-only nodes are not used as demotion targets, which this
> patch set has already addressed.

Yes, I agree. Fixing the bug and allowing override by userspace are
totally two separate things.

>
> > >
> > > To reduce the complexity, the demotion order of the system is either in
> > > overridden mode or automatic mode.  When converting from the automatic
> > > mode to the overridden mode, the existing demotion targets of all nodes
> > > will be retained before being changed.  When converting from overridden
> > > mode to automatic mode, the demotion order of the system will be re-
> > > generated automatically.
> > >
> > > In overridden mode, the demotion targets of the hot-added and hot-
> > > removed node will be set to empty.  And the hot-removed node will be
> > > removed from the demotion targets of any node.
> > >
> > > This is an extention of the interface used in the following patch,
> > >
> > > https://lore.kernel.org/lkml/20191016221149.74AE222C@viggo.jf.intel.com/
> > >
> > > What do you think about this?
> > >
> > > > node state N_DEMOTION_TARGETS is also set from the dax kmem
> > > > driver, certain type of memory which registers through dax kmem
> > > > (e.g. HBM) may not be the right choices for demotion so in future
> > > > they should be distinguished based on certain attributes and dax
> > > > kmem driver should avoid setting them as N_DEMOTION_TARGETS,
> > > > however current implementation also doesn't distinguish any
> > > > such memory and it considers all N_MEMORY as demotion targets
> > > > so this patch series doesn't modify the current behavior.
> > > >
> > >
> > > Best Regards,
> > > Huang, Ying
> > >
> > > [snip]
> > >