Re: [PATCH v4 00/12] sched: Avoid unnecessary migrations within SMT domains

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 06, 2023 at 01:31:36PM -0700, Ricardo Neri wrote:
> Hi,
> 
> This is v4 of this series. Previous versions can be found here [1], [2],
> and here [3]. To avoid duplication, I do not include the cover letter of
> the original submission. You can read it in [1].
> 
> This patchset applies cleanly on today's master branch of the tip tree.
> 
> Changes since v3:
> 
> Nobody liked the proposed changes to the setting of prefer_sibling.
> Instead, I tweaked the solution that Dietmar proposed. Now the busiest
> group, not the local group, determines the setting of prefer_sibling.
> 
> Vincent suggested improvements to the logic to decide whether to follow
> asym_packing priorities. Peter suggested to wrap that in a helper function.
> I added sched_use_asym_prio().
> 
> Ionela found that removing SD_ASYM_PACKING from the SMT domain in x86
> rendered sd_asym_packing NULL in SMT cores. Now highest_flag_domain()
> does not assume that all child domains have the requested flag.
> 
> Tim found that asym_active_balance() needs to also check for the idle
> states of the SMT siblings of lb_env::dst_cpu. I added such check.
> 
> I wrongly assumed that asym_packing could only be used when the busiest
> group had exactly one busy CPU. This broke asym_packing balancing at the
> DIE domain. I limited this check to balances between cores at the MC
> level.
> 
> As per suggestion from Dietmar, I removed sched_asym_smt_can_pull_tasks()
> and placed its logic in sched_asym(). Also, sched_asym() uses
> sched_smt_active() to skip checks when not needed.
> 
> I also added a patch from Chen Yu to enable asym_packing balancing in
> Meteor Lake, which has CPUs of different maximum frequency in more than
> one die.

Is the actual topology of Meteor Lake already public? This patch made me
wonder if we need SCHED_CLUSTER topology in the hybrid_topology thing,
but I can't remember (one of the raisins why the endless calls are such
a frigging waste of time) and I can't seem to find the answer using
Google either.

> Hopefully, these patches are in sufficiently good shape to be merged?

Changelogs are very sparse towards the end and I had to reverse engineer
some of it which is a shame. But yeah, on a first reading the code looks
mostly ok. Specifically 8-10 had me WTF a bit and only at 11 did it
start to make a little sense. Mostly they utterly fail to answer the
very fundament "why did you do this" question.

Also, you seem to have forgotten to Cc our friends from IBM such that
they might verify you didn't break their Power7 stuff -- or do you have
a Power7 yourself to verify and forgot to mention that?

> Chen Yu (1):
>   x86/sched: Add the SD_ASYM_PACKING flag to the die domain of hybrid
>     processors
> 
> Ricardo Neri (11):
>   sched/fair: Move is_core_idle() out of CONFIG_NUMA
>   sched/fair: Only do asym_packing load balancing from fully idle SMT
>     cores
>   sched/fair: Simplify asym_packing logic for SMT cores
>   sched/fair: Let low-priority cores help high-priority busy SMT cores
>   sched/fair: Keep a fully_busy SMT sched group as busiest
>   sched/fair: Use the busiest group to set prefer_sibling
>   sched/fair: Do not even the number of busy CPUs via asym_packing
>   sched/topology: Check SDF_SHARED_CHILD in highest_flag_domain()
>   sched/topology: Remove SHARED_CHILD from ASYM_PACKING
>   x86/sched: Remove SD_ASYM_PACKING from the SMT domain flags
>   x86/sched/itmt: Give all SMT siblings of a core the same priority
> 
>  arch/x86/kernel/itmt.c         |  23 +---
>  arch/x86/kernel/smpboot.c      |   4 +-
>  include/linux/sched/sd_flags.h |   5 +-
>  kernel/sched/fair.c            | 216 +++++++++++++++++----------------
>  kernel/sched/sched.h           |  22 +++-
>  5 files changed, 138 insertions(+), 132 deletions(-)

I'm going to start to queue this and hopefully push out post -rc1 if
nobody objects.