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Subject: Re: [PATCH v2 0/5] mm: demotion: Introduce new node state
 N_DEMOTION_TARGETS
From:   "ying.huang@intel.com" <ying.huang@intel.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Jagdish Gediya <jvgediya@linux.ibm.com>,
        Wei Xu <weixugc@google.com>, Yang Shi <shy828301@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Davidlohr Bueso <dave@stgolabs.net>
Cc:     Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Greg Thelen <gthelen@google.com>,
        MichalHocko <mhocko@kernel.org>,
        Brice Goglin <brice.goglin@gmail.com>
Date:   Mon, 25 Apr 2022 14:10:55 +0800
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On Mon, 2022-04-25 at 09:20 +0530, Aneesh Kumar K.V wrote:
> "ying.huang@intel.com" <ying.huang@intel.com> writes:
> 
> > Hi, All,
> > 
> > On Fri, 2022-04-22 at 16:30 +0530, Jagdish Gediya wrote:
> > 
> > [snip]
> > 
> > > I think it is necessary to either have per node demotion targets
> > > configuration or the user space interface supported by this patch
> > > series. As we don't have clear consensus on how the user interface
> > > should look like, we can defer the per node demotion target set
> > > interface to future until the real need arises.
> > > 
> > > Current patch series sets N_DEMOTION_TARGET from dax device kmem
> > > driver, it may be possible that some memory node desired as demotion
> > > target is not detected in the system from dax-device kmem probe path.
> > > 
> > > It is also possible that some of the dax-devices are not preferred as
> > > demotion target e.g. HBM, for such devices, node shouldn't be set to
> > > N_DEMOTION_TARGETS. In future, Support should be added to distinguish
> > > such dax-devices and not mark them as N_DEMOTION_TARGETS from the
> > > kernel, but for now this user space interface will be useful to avoid
> > > such devices as demotion targets.
> > > 
> > > We can add read only interface to view per node demotion targets
> > > from /sys/devices/system/node/nodeX/demotion_targets, remove
> > > duplicated /sys/kernel/mm/numa/demotion_target interface and instead
> > > make /sys/devices/system/node/demotion_targets writable.
> > > 
> > > Huang, Wei, Yang,
> > > What do you suggest?
> > 
> > We cannot remove a kernel ABI in practice.  So we need to make it right
> > at the first time.  Let's try to collect some information for the kernel
> > ABI definitation.
> > 
> > The below is just a starting point, please add your requirements.
> > 
> > 1. Jagdish has some machines with DRAM only NUMA nodes, but they don't
> > want to use that as the demotion targets.  But I don't think this is a
> > issue in practice for now, because demote-in-reclaim is disabled by
> > default.
> 
> It is not just that the demotion can be disabled. We should be able to
> use demotion on a system where we can find DRAM only NUMA nodes. That
> cannot be achieved by /sys/kernel/mm/numa/demotion_enabled. It needs
> something similar to to N_DEMOTION_TARGETS
> 

Can you show NUMA information of your machines with DRAM-only nodes and
PMEM nodes?  We can try to find the proper demotion order for the
system.  If you can not show it, we can defer N_DEMOTION_TARGETS until
the machine is available.

> > 2. For machines with PMEM installed in only 1 of 2 sockets, for example,
> > 
> > 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
> > 
> > We have 2 choices,
> > 
> > a)
> > node	demotion targets
> > 0	1
> > 2	1
> 
> This is achieved by 
> 
> [PATCH v2 1/5] mm: demotion: Set demotion list differently
> 
> > 
> > b)
> > node	demotion targets
> > 0	1
> > 2	X
> 
> 
> > 
> > a) is good to take advantage of PMEM.  b) is good to reduce cross-socket
> > traffic.  Both are OK as defualt configuration.  But some users may
> > prefer the other one.  So we need a user space ABI to override the
> > default configuration.
> > 
> > 3. For machines with HBM (High Bandwidth Memory), as in
> > 
> > https://lore.kernel.org/lkml/39cbe02a-d309-443d-54c9-678a0799342d@gmail.com/
> > 
> > > [1] local DDR = 10, remote DDR = 20, local HBM = 31, remote HBM = 41
> > 
> > Although HBM has better performance than DDR, in ACPI SLIT, their
> > distance to CPU is longer.  We need to provide a way to fix this.  The
> > user space ABI is one way.  The desired result will be to use local DDR
> > as demotion targets of local HBM.
> 
> 
> IMHO the above (2b and 3) can be done using per node demotion targets. Below is
> what I think we could do with a single slow memory NUMA node 4.

If we can use writable per-node demotion targets as ABI, then we don't
need N_DEMOTION_TARGETS.

> /sys/devices/system/node# cat node[0-4]/demotion_targets
> 4
> 4
> 4
> 4
> 
> /sys/devices/system/node# echo 1 > node1/demotion_targets 
> bash: echo: write error: Invalid argument
> /sys/devices/system/node# cat node[0-4]/demotion_targets
> 4
> 4
> 4
> 4
> 
> /sys/devices/system/node# echo 0 > node1/demotion_targets 
> /sys/devices/system/node# cat node[0-4]/demotion_targets
> 4
> 0
> 4
> 4
> 
> /sys/devices/system/node# echo 1 > node0/demotion_targets 
> bash: echo: write error: Invalid argument
> /sys/devices/system/node# cat node[0-4]/demotion_targets
> 4
> 0
> 4
> 4
> 
> Disable demotion for a specific node.
> /sys/devices/system/node# echo > node1/demotion_targets 
> /sys/devices/system/node# cat node[0-4]/demotion_targets
> 4
> 
> 4
> 4
> 
> Reset demotion to default
> /sys/devices/system/node# echo -1 > node1/demotion_targets 
> /sys/devices/system/node# cat node[0-4]/demotion_targets
> 4
> 4
> 4
> 4
> 
> When a specific device/NUMA node is used for demotion target via the user interface, it is taken
> out of other NUMA node targets.

IMHO, we should be careful about interaction between auto-generated and
overridden demotion order.

Best Regards,
Huang, Ying

> root@ubuntu-guest:/sys/devices/system/node# cat node[0-4]/demotion_targets
> 4
> 4
> 4
> 4
> 
> /sys/devices/system/node# echo 4 > node1/demotion_targets 
> /sys/devices/system/node# cat node[0-4]/demotion_targets
> 
> 4
> 
> 
> 
> If more than one node requies the same demotion target
> /sys/devices/system/node# echo 4 > node0/demotion_targets 
> /sys/devices/system/node# cat node[0-4]/demotion_targets
> 4
> 4
> 
> 
> 
> -aneesh