Re: [PATCH v4 00/10] sched: Instrument sched domain flags

From: Dietmar Eggemann
Date: Thu Aug 06 2020 - 12:49:03 EST

On 31/07/2020 13:54, Valentin Schneider wrote:
> 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)
> 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.


Tested on Arm & Arm64 dual-cluster big.LITTLE (so only
default_topology[]) with CONFIG_SCHED_MC=y for the following cases:

(1) normal bring-up
(2) CPU hp all but one CPU of one cluster
(3) CPU hp entire cluster

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Tested-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>