linux-kernel - Re: [PATCH v3 0/7] mm: demotion: Introduce new node state N_DEMOTION

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Date:   Wed, 27 Apr 2022 09:29:12 +0800
From:   "ying.huang@...el.com" <ying.huang@...el.com>
To:     Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, baolin.wang@...ux.alibaba.com,
        dave.hansen@...ux.intel.com, shy828301@...il.com,
        weixugc@...gle.com, gthelen@...gle.com, dan.j.williams@...el.com
Subject: Re: [PATCH v3 0/7] mm: demotion: Introduce new node state
 N_DEMOTION_TARGETS

On Mon, 2022-04-25 at 20:14 +0530, Aneesh Kumar K V wrote:
> On 4/25/22 7:27 PM, Jonathan Cameron wrote:
> > On Mon, 25 Apr 2022 16:45:38 +0530
> > Jagdish Gediya <jvgediya@...ux.ibm.com> wrote:
> > 
> > > On Sun, Apr 24, 2022 at 11:19:53AM +0800, ying.huang@...el.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.
> > 
> > I'm not convinced the distinction between DRAM and persistent memory is
> > valid. There will definitely be systems with a large pool
> > of remote DRAM (and potentially no NV memory) where the right choice
> > is to demote to that DRAM pool.
> > 
> > Basing the decision on whether the memory is from kmem or
> > normal DRAM doesn't provide sufficient information to make the decision.
> > 
> 
> Hence the suggestion for the ability to override this from userspace. 
> Now, for example, we could build a system with memory from the remote 
> machine (memory inception in case of power which will mostly be plugged 
> in as regular hotpluggable memory ) and a slow CXL memory or OpenCAPI 
> memory.
> 
> In the former case, we won't consider that for demotion with this series 
> because that is not instantiated via dax kmem. So yes definitely we 
> would need the ability to override this from userspace so that we could 
> put these remote memory NUMA nodes as demotion targets if we want.
> > > 

Is there a driver for the device (memory from the remote machine)?  If
so, we can adjust demotion order for it in the driver.

In general, I think that we can adjust demotion order inside kernel from
various information sources.  In addition to ACPI SLIT, we also have
HMAT, kmem driver, other drivers, etc.

> > 
Best Regards,
Huang, Ying