RE: [RFC PATCH v3 0/2] scheduler: expose the topology of clusters and add cluster scheduler

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Tim Chen [mailto:tim.c.chen@xxxxxxxxxxxxxxx]
> Sent: Friday, January 8, 2021 12:17 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>;
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> Subject: Re: [RFC PATCH v3 0/2] scheduler: expose the topology of clusters and
> add cluster scheduler
> 
> 
> 
> On 1/6/21 12:30 AM, Barry Song wrote:
> > ARM64 server chip Kunpeng 920 has 6 clusters in each NUMA node, and each
> > cluster has 4 cpus. All clusters share L3 cache data while each cluster
> > has local L3 tag. On the other hand, each cluster will share some
> > internal system bus. This means cache is much more affine inside one cluster
> > than across clusters.
> >
> >     +-----------------------------------+                          +---------+
> >     |  +------+    +------+            +---------------------------+         |
> >     |  | CPU0 |    | cpu1 |             |    +-----------+         |         |
> >     |  +------+    +------+             |    |           |         |         |
> >     |                                   +----+    L3     |         |         |
> >     |  +------+    +------+   cluster   |    |    tag    |         |         |
> >     |  | CPU2 |    | CPU3 |             |    |           |         |         |
> >     |  +------+    +------+             |    +-----------+         |         |
> >     |                                   |                          |         |
> >     +-----------------------------------+                          |         |
> >     +-----------------------------------+                          |         |
> >     |  +------+    +------+             +--------------------------+         |
> >     |  |      |    |      |             |    +-----------+         |         |
> >     |  +------+    +------+             |    |           |         |         |
> >     |                                   |    |    L3     |         |         |
> >     |  +------+    +------+             +----+    tag    |         |         |
> >     |  |      |    |      |             |    |           |         |         |
> >     |  +------+    +------+             |    +-----------+         |         |
> >     |                                   |                          |         |
> >     +-----------------------------------+                          |   L3    |
> >                                                                    |   data  |
> >     +-----------------------------------+                          |         |
> >     |  +------+    +------+             |    +-----------+         |         |
> >     |  |      |    |      |             |    |           |         |         |
> >     |  +------+    +------+             +----+    L3     |         |         |
> >     |                                   |    |    tag    |         |         |
> >     |  +------+    +------+             |    |           |         |         |
> >     |  |      |    |      |            ++    +-----------+         |         |
> >     |  +------+    +------+            |---------------------------+         |
> >     +-----------------------------------|                          |         |
> >     +-----------------------------------|                          |         |
> >     |  +------+    +------+            +---------------------------+         |
> >     |  |      |    |      |             |    +-----------+         |         |
> >     |  +------+    +------+             |    |           |         |         |
> >     |                                   +----+    L3     |         |         |
> >     |  +------+    +------+             |    |    tag    |         |         |
> >     |  |      |    |      |             |    |           |         |         |
> >     |  +------+    +------+             |    +-----------+         |         |
> >     |                                   |                          |         |
> >     +-----------------------------------+                          |         |
> >     +-----------------------------------+                          |         |
> >     |  +------+    +------+             +--------------------------+         |
> >     |  |      |    |      |             |   +-----------+          |         |
> >     |  +------+    +------+             |   |           |          |         |
> >
> >
> 
> There is a similar need for clustering in x86.  Some x86 cores could share L2
> caches that
> is similar to the cluster in Kupeng 920 (e.g. on Jacobsville there are 6 clusters
> of 4 Atom cores, each cluster sharing a separate L2, and 24 cores sharing L3).
> Having a sched domain at the L2 cluster helps spread load among
> L2 domains.  This will reduce L2 cache contention and help with
> performance for low to moderate load scenarios.
> 
> The cluster detection mechanism will need
> to be based on L2 cache sharing in this case.  I suggest making the
> cluster detection to be CPU architecture dependent so both ARM64 and x86 use
> cases
> can be accommodated.
> 
> Attached below are two RFC patches for creating x86 L2
> cache sched domain, sans the idle cpu selection on wake up code.  It is
> similar enough in concept to Barry's patch that we should have a
> single patchset that accommodates both use cases.

Hi Tim, Agreed on this.
hopefully the RFC v4 I am preparing will cover your case.

> 
> Thanks.
> 
> Tim

Thanks
Barry