Date:   Fri, 31 Jul 2020 12:54:52 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, peterz@...radead.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, morten.rasmussen@....com,
        Quentin Perret <qperret@...gle.com>
Subject: [PATCH v4 00/10] sched: Instrument sched domain flags

Hi,

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.

Patches
=======

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.

o Patches 1-3 are topology cleanups / fixes
o Patches 4-6 instrument SD flags and add assertions
o Patches 7-10 leverage the instrumentation to factorize domain degeneration

Revisions
=========

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)

o Made SD_NUMA and SD_SERIALIZE have SDF_NEEDS_GROUPS

  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 (10):
  ARM, sched/topology: Remove SD_SHARE_POWERDOMAIN
  ARM: Revert back to default scheduler topology.
  sched/topology: Propagate SD_ASYM_CPUCAPACITY upwards
  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/topology: Add more flags to the SD degeneration mask
  sched/topology: Remove SD_SERIALIZE degeneration special case
  sched/topology: Introduce SD metaflag for flags needing > 1 groups
  sched/topology: Use prebuilt SD flag degeneration mask

 arch/arm/kernel/topology.c     |  26 ------
 include/linux/sched/sd_flags.h | 156 +++++++++++++++++++++++++++++++++
 include/linux/sched/topology.h |  36 +++++---
 kernel/sched/topology.c        |  54 ++++++------
 4 files changed, 204 insertions(+), 68 deletions(-)
 create mode 100644 include/linux/sched/sd_flags.h

--
2.27.0

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