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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:     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, aneesh.kumar@linux.ibm.com,
        shy828301@gmail.com, weixugc@google.com, gthelen@google.com,
        dan.j.williams@intel.com
Date:   Tue, 26 Apr 2022 15:55:36 +0800
In-Reply-To: <YmaC2jw6WaQ4X+8W@li-6e1fa1cc-351b-11b2-a85c-b897023bb5f3.ibm.com>
References: <20220422195516.10769-1-jvgediya@linux.ibm.com>
         <4b986b46afb2fe888c127d8758221d0f0d3ec55f.camel@intel.com>
         <YmaC2jw6WaQ4X+8W@li-6e1fa1cc-351b-11b2-a85c-b897023bb5f3.ibm.com>
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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.

> 2) Once this new node brought online,  without N_DEMOTION_TARGETS
> patch series, 1 is demotion target for 0 and 2 is demotion target
> for 1.
> 
> With this patch series applied,
> 1) No demotion target for either 0 or 1 before dax device is online
> 2) 2 is demotion target for both 0 and 1 after dax device is online.
> 

So with your change, if a node hasn't N_DEMOTION_TARGETS, it will become
a top-level demotion source even if it hasn't N_CPU?  If so, I cannot
clear N_DEMOTION_TARGETS for a node in middle or bottom level?

Best Regards,
Huang, Ying

> > 
[snip]