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From:   Yang Shi <shy828301@gmail.com>
Date:   Wed, 20 Apr 2022 20:11:59 -0700
Message-ID: <CAHbLzkpGzEaSDfM=GBzBxw=dZTBy12vgDDhMG+q4dbG+bCgR6A@mail.gmail.com>
Subject: Re: [PATCH v2 0/5] mm: demotion: Introduce new node state N_DEMOTION_TARGETS
To:     "ying.huang@intel.com" <ying.huang@intel.com>
Cc:     Jagdish Gediya <jvgediya@linux.ibm.com>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Wei Xu <weixugc@google.com>
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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.

>
> 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]
>