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Subject: Re: [PATCH v3 0/7] 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>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        akpm@linux-foundation.org, baolin.wang@linux.alibaba.com,
        dave.hansen@linux.intel.com, shy828301@gmail.com,
        weixugc@google.com, gthelen@google.com, dan.j.williams@intel.com
Date:   Tue, 26 Apr 2022 17:10:04 +0800
In-Reply-To: <63b0c4ab-861c-3d07-4912-e6cd842d0bfd@linux.ibm.com>
References: <20220422195516.10769-1-jvgediya@linux.ibm.com>
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         <085260285e48093f48d889994aaa500a78577bf2.camel@intel.com>
         <63b0c4ab-861c-3d07-4912-e6cd842d0bfd@linux.ibm.com>
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On Tue, 2022-04-26 at 14:37 +0530, Aneesh Kumar K V wrote:
> On 4/26/22 1:25 PM, ying.huang@intel.com wrote:
> > On Mon, 2022-04-25 at 16:45 +0530, Jagdish Gediya wrote:
> > > On Sun, Apr 24, 2022 at 11:19:53AM +0800, ying.huang@intel.com wrote:
> > > > On Sat, 2022-04-23 at 01:25 +0530, Jagdish Gediya wrote:
> > > > > Some systems(e.g. PowerVM) can have both DRAM(fast memory) only
> > > > > NUMA node which are N_MEMORY and slow memory(persistent memory)
> > > > > only NUMA node which are also N_MEMORY. As the current demotion
> > > > > target finding algorithm works based on N_MEMORY and best distance,
> > > > > it will choose DRAM only NUMA node as demotion target instead of
> > > > > persistent memory node on such systems. If DRAM only NUMA node is
> > > > > filled with demoted pages then at some point new allocations can
> > > > > start falling to persistent memory, so basically cold pages are in
> > > > > fast memor (due to demotion) and new pages are in slow memory, this
> > > > > is why persistent memory nodes should be utilized for demotion and
> > > > > dram node should be avoided for demotion so that they can be used
> > > > > for new allocations.
> > > > > 
> > > > > Current implementation can work fine on the system where the memory
> > > > > only numa nodes are possible only for persistent/slow memory but it
> > > > > is not suitable for the like of systems mentioned above.
> > > > 
> > > > Can you share the NUMA topology information of your machine?  And the
> > > > demotion order before and after your change?
> > > > 
> > > > Whether it's good to use the PMEM nodes as the demotion targets of the
> > > > DRAM-only node too?
> > > 
> > > $ numactl -H
> > > available: 2 nodes (0-1)
> > > node 0 cpus: 0 1 2 3 4 5 6 7
> > > node 0 size: 14272 MB
> > > node 0 free: 13392 MB
> > > node 1 cpus:
> > > node 1 size: 2028 MB
> > > node 1 free: 1971 MB
> > > node distances:
> > > node   0   1
> > >    0:  10  40
> > >    1:  40  10
> > > 
> > > 1) without N_DEMOTION_TARGETS patch series, 1 is demotion target
> > >     for 0 even when 1 is DRAM node and there is no demotion targets for 1.
> > > 
> > > $ cat /sys/bus/nd/devices/dax0.0/target_node
> > > 2
> > > $
> > > # cd /sys/bus/dax/drivers/
> > > :/sys/bus/dax/drivers# ls
> > > device_dax  kmem
> > > :/sys/bus/dax/drivers# cd device_dax/
> > > :/sys/bus/dax/drivers/device_dax# echo dax0.0 > unbind
> > > :/sys/bus/dax/drivers/device_dax# echo dax0.0 >  ../kmem/new_id
> > > :/sys/bus/dax/drivers/device_dax# numactl -H
> > > available: 3 nodes (0-2)
> > > node 0 cpus: 0 1 2 3 4 5 6 7
> > > node 0 size: 14272 MB
> > > node 0 free: 13380 MB
> > > node 1 cpus:
> > > node 1 size: 2028 MB
> > > node 1 free: 1961 MB
> > > node 2 cpus:
> > > node 2 size: 0 MB
> > > node 2 free: 0 MB
> > > node distances:
> > > node   0   1   2
> > >    0:  10  40  80
> > >    1:  40  10  80
> > >    2:  80  80  10
> > > 
> > 
> > This looks like a virtual machine, not a real machine.  That's
> > unfortunate.  I am looking forward to a real issue, not a theoritical
> > possible issue.
> > 
> 
> This is the source of confusion i guess. A large class of ppc64 systems 
> are virtualized. The firmware include a hypervisor (PowerVM) and end 
> user creates guest (aka LPAR) on them. That is the way end user will use 
> this system. There is no baremetal access on this (There is an openpower 
> variant, but all new systems built by IBM these days do have PowerVM on 
> them).
> 
> 
> So this is not a theoretical possibility.
> 

Now I get it.  Thanks for detailed explanation.

Best Regards,
Huang, Ying