On Sat, Oct 2, 2021 at 12:22 PM Tim Chen [off-list ref] wrote:

On Fri, 2021-10-01 at 16:57 +0200, Peter Zijlstra wrote:

quoted

On Fri, Oct 01, 2021 at 12:39:56PM +0200, Vincent Guittot wrote:

quoted

Hi Barry,

On Fri, 1 Oct 2021 at 12:32, Barry Song [off-list ref] wrote:

quoted

Hi Vincent, Dietmar, Peter, Ingo,
Do you have any comment on this first series which exposes
cluster topology
of ARM64 kunpeng 920 & x86 Jacobsville and supports load balance
only for
the 1st stage?
I will be very grateful for your comments so that things can move
forward in the
right direction. I think Tim also looks forward to bringing up
cluster
support in
Jacobsville.

This patchset makes sense to me and the addition of a new
scheduling
level to better reflect the HW topology goes in the right
direction.

So I had a look, dreading the selecti-idle-sibling changes, and was
pleasantly surprised they're gone :-)

Thanks, Peter and Vincent for reviewing.

My tiny scheduler team is still hardly working on the
select-idle-sibling changes.
And that one will be sent as a separate series as an improvement to this series.
I promise the wake-affine series won't be that scary when you see it
next time :-)

quoted

As is, this does indeed look like something mergable without too much
hassle.

The one questino I have is, do we want default y?

I also agree that default y is preferable.

Thanks, Tim, for your comments.
I am ok to make it default "Y" for x86 after having a better doc as below:

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index bd27b1cdac34..940eb1fe0abb 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig

@@ -1002,12 +1002,17 @@ config NR_CPUS
          to the kernel image.

 config SCHED_CLUSTER
-       bool "Cluster scheduler support"
-       default n
+       def_bool y
+       prompt "Cluster scheduler support"
        help
         Cluster scheduler support improves the CPU scheduler's decision
-        making when dealing with machines that have clusters of CPUs
-        sharing L2 cache. If unsure say N here.
+        making when dealing with machines that have clusters of CPUs.
+        Cluster usually means a couple of CPUs which are placed closely
+        by sharing mid-level caches, last-level cache tags or internal
+        busses. For example, on x86 Jacobsville, each 4 CPUs share one
+        L2 cache. This feature isn't a universal win because it can bring
+        a cost of slightly increased overhead in some places. If unsure
+        say N here.

This also aligns well with SCHED_MC and SCHED_SMT in arch/x86/kconfig:
config SCHED_MC
    def_bool y
    prompt "Multi-core scheduler support"

config SCHED_SMT
    def_bool y if SMP

But ARM64 is running in a different tradition, arch/arm64/Kconfig has
SCHED_MC and SCHED_SMT as below:
   config SCHED_MC
   bool "Multi-core scheduler support"
   help
    ...

config SCHED_SMT
  bool "SMT scheduler support"
  help
  ...

I don't want to be an odd man :-)  So for ARM64, I vote keeping the
Kconfig file as is.  And I am planning to modify arch/arm64/defconfig
in second patchset(select-idle-sibling) by adding
CONFIG_SCHED_CLUSTR=y
as load-balance plus wake-affine changes seem to make cluster
scheduler much more widely win on kunpeng920 while doing load-
balance only can sometimes hurt. so I don't mind holding "N" for
a while on the ARM64 platform.

quoted

The one nit I have is the Kconfig text, I'm not really sure that's
clarifying what a cluster is.

Do you have a preference of a different name other than cluster?
Or simply better documentation on what a cluster is for ARM64
and x86 in Kconfig?

Anyway, naming is really a hard thing. cluster seems not a bad name for
ARM SoCs as besides kunpeng, some other ARM SoCs are also using this
name in specifications, for example, neoverse-n1, phytium etc.

Will we use the same name between x86 and ARM and just refine the document
as below? Does the below doc explain what is "cluster" better?

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 7e4651a1aaf4..86821e83b935 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig

@@ -993,8 +993,13 @@ config SCHED_CLUSTER
        bool "Cluster scheduler support"
        help
          Cluster scheduler support improves the CPU scheduler's decision
-         making when dealing with machines that have clusters(sharing internal
-         bus or sharing LLC cache tag). If unsure say N here.
+         making when dealing with machines that have clusters of CPUs.
+         Cluster usually means a couple of CPUs which are placed closely
+         by sharing mid-level caches, last-level cache tags or internal
+         busses. For example, on Hisilicon Kunpeng920, each 4 CPUs share
+         LLC cache tags. This feature isn't a universal win because it
+         can bring a cost of slightly increased overhead in some places.
+         If unsure say N here.

 config SCHED_SMT
        bool "SMT scheduler support"

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index bd27b1cdac34..940eb1fe0abb 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig

@@ -1002,12 +1002,17 @@ config NR_CPUS
          to the kernel image.

 config SCHED_CLUSTER
-       bool "Cluster scheduler support"
-       default n
+       def_bool y
+       prompt "Cluster scheduler support"
        help
         Cluster scheduler support improves the CPU scheduler's decision
-        making when dealing with machines that have clusters of CPUs
-        sharing L2 cache. If unsure say N here.
+        making when dealing with machines that have clusters of CPUs.
+        Cluster usually means a couple of CPUs which are placed closely
+        by sharing mid-level caches, last-level cache tags or internal
+        busses. For example, on x86 Jacobsville, each 4 CPUs share one
+        L2 cache. This feature isn't a universal win because it can bring
+        a cost of slightly increased overhead in some places. If unsure
+        say N here.

 config SCHED_SMT
        def_bool y if SMP

Thanks.

Tim

Thanks
barry

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