linux-kernel - Re: [PATCH -V10 RESEND 0/6] NUMA balancing: optimize memory placement for memory tiering system

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Date:   Thu, 13 Jan 2022 10:49:35 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Feng Tang <feng.tang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Rik van Riel <riel@...riel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>,
        Wei Xu <weixugc@...gle.com>, osalvador <osalvador@...e.de>,
        Shakeel Butt <shakeelb@...gle.com>,
        Hasan Al Maruf <hasanalmaruf@...com>
Subject: Re: [PATCH -V10 RESEND 0/6] NUMA balancing: optimize memory
 placement for memory tiering system

On Thu, Jan 13, 2022 at 03:19:06PM +0800, Huang, Ying wrote:
> Hi, Peter,
> 
> Peter Zijlstra <peterz@...radead.org> writes:
> 
> > On Tue, Dec 07, 2021 at 10:27:51AM +0800, Huang Ying wrote:
> >> After commit c221c0b0308f ("device-dax: "Hotplug" persistent memory
> >> for use like normal RAM"), the PMEM could be used as the
> >> cost-effective volatile memory in separate NUMA nodes.  In a typical
> >> memory tiering system, there are CPUs, DRAM and PMEM in each physical
> >> NUMA node.  The CPUs and the DRAM will be put in one logical node,
> >> while the PMEM will be put in another (faked) logical node.
> >
> > So what does a system like that actually look like, SLIT table wise, and
> > how does that affect init_numa_topology_type() ?
> 
> The SLIT table is as follows,
> 
> [000h 0000   4]                    Signature : "SLIT"    [System Locality Information Table]
> [004h 0004   4]                 Table Length : 0000042C
> [008h 0008   1]                     Revision : 01
> [009h 0009   1]                     Checksum : 59
> [00Ah 0010   6]                       Oem ID : "INTEL "
> [010h 0016   8]                 Oem Table ID : "S2600WF "
> [018h 0024   4]                 Oem Revision : 00000001
> [01Ch 0028   4]              Asl Compiler ID : "INTL"
> [020h 0032   4]        Asl Compiler Revision : 20091013
> 
> [024h 0036   8]                   Localities : 0000000000000004
> [02Ch 0044   4]                 Locality   0 : 0A 15 11 1C
> [030h 0048   4]                 Locality   1 : 15 0A 1C 11
> [034h 0052   4]                 Locality   2 : 11 1C 0A 1C
> [038h 0056   4]                 Locality   3 : 1C 11 1C 0A
> 
> The `numactl -H` output is as follows,
> 
> available: 4 nodes (0-3)
> node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
> node 0 size: 64136 MB
> node 0 free: 5981 MB
> node 1 cpus: 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
> node 1 size: 64466 MB
> node 1 free: 10415 MB
> node 2 cpus:
> node 2 size: 253952 MB
> node 2 free: 253920 MB
> node 3 cpus:
> node 3 size: 253952 MB
> node 3 free: 253951 MB
> node distances:
> node   0   1   2   3 
>   0:  10  21  17  28 
>   1:  21  10  28  17 
>   2:  17  28  10  28 
>   3:  28  17  28  10 
> 
> init_numa_topology_type() set sched_numa_topology_type to NUMA_DIRECT.
> 
> The node 0 and node 1 are onlined during boot.  While the PMEM node,
> that is, node 2 and node 3 are onlined later.  As in the following dmesg
> snippet.

But how? sched_init_numa() scans the *whole* SLIT table to determine
nr_levels / sched_domains_numa_levels, even offline nodes. Therefore it
should find 4 distinct distance values and end up not selecting
NUMA_DIRECT.

Similarly for the other types it uses for_each_online_node(), which
would include the pmem nodes once they've been onlined, but I'm thinking
we explicitly want to skip CPU-less nodes in that iteration.