Re: [PATCH v4 1/3] mm/damon: Introduce DAMOS_QUOTA_HUGEPAGE auto tuning

[Lian Wang <lianux.mm@gmail.com>]

Hi SJ and Asier,

Thanks for working on this and for providing the server results.  I have two
questions about what hugepage_mem_bp is intended to represent.

> Introduce DAMOS_QUOTA_HUGEPAGE_MEM_BP auto tuning. Add a new DAMOS quota
> goal metric to measure the amount of huge page consumption to total
> memory consumption ratio.

First, NR_ANON_THPS tracks anonymous PMD mappings, while NR_SHMEM_THPS and
NR_FILE_THPS track PMD-mappable page-cache folios.  Therefore, splitting only
an anonymous PMD mapping can change hugepage_mem_bp without physically
splitting the folio.  Is the intended metric PMD-mapped memory or physical
large-folio memory?  Clarifying this and adding a mapping-only test may help.

Second, the metric is global while a DAMOS scheme can target one process.
THPs from other processes or NUMA nodes can satisfy the target or dilute the
monitored process's changes.  Is this intentional?  If so, documenting the
scope and testing a background THP workload may be useful.

The temporal results approach the 10% and 25% targets, while the consistent
results overshoot the 10% target to about 20% and 45%.  I would describe this
as control-response data.  TPS, latency, TLB, fragmentation and collapse CPU
data could further show the workload benefit and cost.

If I have misunderstood any of this, please feel free to ignore these
comments and correct me.

The enum, quota-goal wiring and sysfs exposure otherwise look consistent with
the existing DAMOS autotuning framework.  I consider the points above
follow-up questions about semantics and evaluation, rather than blockers for
this series.

For the series:

Reviewed-by: Lian Wang <lianux.mm@gmail.com>

Please feel free to Cc me on related follow-up patches.  I am happy to help
review the code.  Besides my own DAMON work, I have recently been reviewing
and learning from other DAMON and MM work, and I would be glad to continue.

Thanks,
Lian

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