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Date:   Mon, 25 Apr 2022 12:56:51 +0530
From:   Jagdish Gediya <jvgediya@linux.ibm.com>
To:     "ying.huang@intel.com" <ying.huang@intel.com>
Cc:     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>,
        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>,
        Greg Thelen <gthelen@google.com>,
        MichalHocko <mhocko@kernel.org>,
        Brice Goglin <brice.goglin@gmail.com>
Subject: Re: [PATCH v2 0/5] mm: demotion: Introduce new node state
 N_DEMOTION_TARGETS
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On Sun, Apr 24, 2022 at 11:02:47AM +0800, ying.huang@intel.com wrote:
> 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.

/sys/kernel/mm/numa/demotion_target was introduced in v2, I was
talking about removing it from next version of the series as the
similar interface is available as a result of introducing
N_DEMOTION_TARGETS at /sys/devices/system/node/demotion_targets, so
instead of introducing duplicate interface to write N_DEMOTION_TARGETS,
we can instead make /sys/devices/system/node/demotion_targets writable.

> 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.
> 
> 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
> 
> 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.
> 
> Best Regards,
> Huang, Ying
> 
>