[PATCH v6 00/17] sched: Instrument sched domain flags

Valentin Schneider
2020-08-17T11:30:16Z

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. 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
degeneration

Revisions
=========

v5 -> v6
--------

o Fixed kernelbot warnings
o Tweaked ARM patches changelogs (Ingo)

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
patches.

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
(Ingo)
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
changed.
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)

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 (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 | 56 +++++++++++-
kernel/sched/topology.c | 54 ++++++------
5 files changed, 268 insertions(+), 69 deletions(-)
create mode 100644 include/linux/sched/sd_flags.h

--
2.27.0