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

[Shrikanth Hegde <sshegde@linux.vnet.ibm.com>]

On 5/19/23 5:33 AM, Ricardo Neri wrote:
> On Fri, May 12, 2023 at 11:53:48PM +0530, Shrikanth Hegde wrote:
>>
>>
>> On 4/29/23 9:02 PM, 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.
>>>
>>>> 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?
>>
>> Very good patch series in addressing asym packing. Interesting discussions as
>> well. Took me quite sometime to get through to understand and do a little bit
>> of testing.
>>
>> Tested this patch a bit on power7 with qemu. Tested with SMT=4. sched domains
>> show ASYM_PACKING present only for SMT domain.
>>
>> We don't see any regressions/gain due to patch. SMT priorities are honored when
>> tasks are scheduled and load_balanced.
> 
> Thank you very much for your review and testing! Would you mind sharing the
> qemu command you use? I would like to test my future patches on power7.

Sure. I tried this qemu command on a Power9 system. It loads in compat mode.
This would simulate 4Core and SMT4 
you need the have CONFIG_POWER7_CPU=y and CONFIG_CPU_BIG_ENDIAN=y and right
qcow2 image. 

qemu-system-ppc64 -M pseries,accel=kvm,max-cpu-compat=power7 -m 8192 -smp cores=4,threads=4 -enable-kvm  -drive file=<qcow2_image>,format=qcow2 -vga none -nographic -net nic -net user,hostfwd=tcp::2222-:22 -kernel ./<custom_vmlinux> --append "noautotest root=/dev/sda2"


> 
> BR,
> Ricardo