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Date:   Wed, 12 Aug 2020 13:52:43 +0100
From:   Valentin Schneider <>
        Quentin Perret <>
Subject: [PATCH v5 00/17] sched: Instrument sched domain flags


I've repeatedly stared at an SD flag and asked myself "how should that be
set up in the domain hierarchy anyway?". I figured that if we formalize our
flags zoology a bit, we could also do some runtime assertions on them -
this is what this series is all about.


The idea is to associate the flags with metaflags that describes how they
should be set in a sched domain hierarchy ("if this SD has it, all its {parents,
children} have it") or how they behave wrt degeneration - details are in the
comments and commit logs. 

The good thing is that the debugging bits go away when CONFIG_SCHED_DEBUG isn't
set. The bad thing is that this replaces SD_* flags definitions with some
unsavoury macros. This is mainly because I wanted to avoid having to duplicate
work between declaring the flags and declaring their metaflags. Conceptually
they are pretty close to the macros used for SCHED_FEAT.

o Patches 1-2 remove a derelict flag and align the arm scheduler topology
  with arm64's
o Patches 3-8 instrument SD flags with metadata and add assertions
o Patches 9-10 are additional topology cleanups
o Patches 11-16 each add a new flag to SD_DEGENERATE_GROUPS_MASK
o Patch 17 leverage the previous 6 patches to further factorize domain


v4 -> v5

The final git diff between v4 and v5 isn't too big; there is no diff on the
domain degeneration side of things, it has just been split up in more

o Shuffled the series around to facilitate bisection (Ingo)
  I kept the arm bits at the start because that was a bit simpler, and in
  the unlikely case that needs to be reverted it won't be too hard.
o Split the degeneration mask tweak into individual commits per new flag
o Collected Reviewed-by from Dietmar; since I shuffled the whole lot I
  didn't keep your Tested-by, sorry!

o Turned the SD flags into an enum with automagic power of 2 assignment.
  I poked the dwarves and they assured me the SD flag values haven't
o [new patch] Made SD flag debug file output flag names

v3 -> v4

o Reordered the series to have fixes / cleanups first

o Added SD_ASYM_CPUCAPACITY propagation (Quentin)
o Made ARM revert back to the default sched topology (Dietmar)
o Removed SD_SERIALIZE degeneration special case (Peter)


  As discussed on v3, I thought this wasn't required, but thinking some more
  about it there can be cases where that changes the current behaviour. For
  instance, in the following wacky triangle:

      0\ 30
      | \
  20  |  2
      | /
      1/ 30

  there are two unique distances thus two NUMA topology levels, however the
  first one for node 2 would have the same span as its child domain and thus
  should be degenerated. If we don't give SD_NUMA and SD_SERIALIZE
  SDF_NEEDS_GROUPS, this domain wouldn't be denegerated since its child
  *doesn't* have either SD_NUMA or SD_SERIALIZE (it's the first NUMA domain),
  and we'd have this weird NUMA domain lingering with a single group.

v2 -> v3

o Reworded comment for SD_OVERLAP (it's about the groups, not the domains)

o Added more flags to the SD degeneration mask
o Added generation of an SD flag mask for the degeneration functions (Peter)

RFC -> v2

o Rebased on top of tip/sched/core
o Aligned wording of comments between flags
o Rectified some flag descriptions (Morten)
o Added removal of SD_SHARE_POWERDOMAIN (Morten)

Valentin Schneider (17):
  ARM, sched/topology: Remove SD_SHARE_POWERDOMAIN
  ARM: Revert back to default scheduler topology.
  sched/topology: Split out SD_* flags declaration to its own file
  sched/topology: Define and assign sched_domain flag metadata
  sched/topology: Verify SD_* flags setup when sched_debug is on
  sched/debug: Output SD flag names rather than their values
  sched/topology: Introduce SD metaflag for flags needing > 1 groups
  sched/topology: Use prebuilt SD flag degeneration mask
  sched/topology: Remove SD_SERIALIZE degeneration special case
  sched/topology: Propagate SD_ASYM_CPUCAPACITY upwards
  sched/topology: Mark SD_PREFER_SIBLING as SDF_NEEDS_GROUPS
  sched/topology: Mark SD_BALANCE_WAKE as SDF_NEEDS_GROUPS
  sched/topology: Mark SD_SERIALIZE as SDF_NEEDS_GROUPS
  sched/topology: Mark SD_ASYM_PACKING as SDF_NEEDS_GROUPS
  sched/topology: Mark SD_OVERLAP as SDF_NEEDS_GROUPS
  sched/topology: Mark SD_NUMA as SDF_NEEDS_GROUPS
  sched/topology: Expand use of SD_DEGENERATE_GROUPS_MASK to flags not
    needing groups

 arch/arm/kernel/topology.c     |  26 ------
 include/linux/sched/sd_flags.h | 156 +++++++++++++++++++++++++++++++++
 include/linux/sched/topology.h |  45 +++++++---
 kernel/sched/debug.c           |  53 ++++++++++-
 kernel/sched/topology.c        |  54 ++++++------
 5 files changed, 265 insertions(+), 69 deletions(-)
 create mode 100644 include/linux/sched/sd_flags.h


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