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

[Tim Chen]

On Sat, 2023-04-29 at 17:32 +0200, Peter Zijlstra wrote:
> 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.

There are a bunch of fixes that are needed for SCHED_CLUSTER to work
properly on hybrid_topology.  I'll clean them up and post them on
top of Ricardo's current patch set this week.

Tim

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