Date:   Tue, 22 Aug 2023 09:52:43 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org,
        nvdimm@...ts.linux.dev, linux-acpi@...r.kernel.org,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Wei Xu <weixugc@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Michal Hocko <mhocko@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Rafael J Wysocki <rafael.j.wysocki@...el.com>,
        Dave Jiang <dave.jiang@...el.com>
Subject: Re: [PATCH RESEND 1/4] memory tiering: add abstract distance
 calculation algorithms management


"Huang, Ying" <ying.huang@...el.com> writes:

> Alistair Popple <apopple@...dia.com> writes:
>
>> "Huang, Ying" <ying.huang@...el.com> writes:
>>
>>> Hi, Alistair,
>>>
>>> Sorry for late response.  Just come back from vacation.
>>
>> Ditto for this response :-)
>>
>> I see Andrew has taken this into mm-unstable though, so my bad for not
>> getting around to following all this up sooner.
>>
>>> Alistair Popple <apopple@...dia.com> writes:
>>>
>>>> "Huang, Ying" <ying.huang@...el.com> writes:
>>>>
>>>>> Alistair Popple <apopple@...dia.com> writes:
>>>>>
>>>>>> "Huang, Ying" <ying.huang@...el.com> writes:
>>>>>>
>>>>>>> Alistair Popple <apopple@...dia.com> writes:
>>>>>>>
>>>>>>>>>>> While other memory device drivers can use the general notifier chain
>>>>>>>>>>> interface at the same time.
>>>>>>>>
>>>>>>>> How would that work in practice though? The abstract distance as far as
>>>>>>>> I can tell doesn't have any meaning other than establishing preferences
>>>>>>>> for memory demotion order. Therefore all calculations are relative to
>>>>>>>> the rest of the calculations on the system. So if a driver does it's own
>>>>>>>> thing how does it choose a sensible distance? IHMO the value here is in
>>>>>>>> coordinating all that through a standard interface, whether that is HMAT
>>>>>>>> or something else.
>>>>>>>
>>>>>>> Only if different algorithms follow the same basic principle.  For
>>>>>>> example, the abstract distance of default DRAM nodes are fixed
>>>>>>> (MEMTIER_ADISTANCE_DRAM).  The abstract distance of the memory device is
>>>>>>> in linear direct proportion to the memory latency and inversely
>>>>>>> proportional to the memory bandwidth.  Use the memory latency and
>>>>>>> bandwidth of default DRAM nodes as base.
>>>>>>>
>>>>>>> HMAT and CDAT report the raw memory latency and bandwidth.  If there are
>>>>>>> some other methods to report the raw memory latency and bandwidth, we
>>>>>>> can use them too.
>>>>>>
>>>>>> Argh! So we could address my concerns by having drivers feed
>>>>>> latency/bandwidth numbers into a standard calculation algorithm right?
>>>>>> Ie. Rather than having drivers calculate abstract distance themselves we
>>>>>> have the notifier chains return the raw performance data from which the
>>>>>> abstract distance is derived.
>>>>>
>>>>> Now, memory device drivers only need a general interface to get the
>>>>> abstract distance from the NUMA node ID.  In the future, if they need
>>>>> more interfaces, we can add them.  For example, the interface you
>>>>> suggested above.
>>>>
>>>> Huh? Memory device drivers (ie. dax/kmem.c) don't care about abstract
>>>> distance, it's a meaningless number. The only reason they care about it
>>>> is so they can pass it to alloc_memory_type():
>>>>
>>>> struct memory_dev_type *alloc_memory_type(int adistance)
>>>>
>>>> Instead alloc_memory_type() should be taking bandwidth/latency numbers
>>>> and the calculation of abstract distance should be done there. That
>>>> resovles the issues about how drivers are supposed to devine adistance
>>>> and also means that when CDAT is added we don't have to duplicate the
>>>> calculation code.
>>>
>>> In the current design, the abstract distance is the key concept of
>>> memory types and memory tiers.  And it is used as interface to allocate
>>> memory types.  This provides more flexibility than some other interfaces
>>> (e.g. read/write bandwidth/latency).  For example, in current
>>> dax/kmem.c, if HMAT isn't available in the system, the default abstract
>>> distance: MEMTIER_DEFAULT_DAX_ADISTANCE is used.  This is still useful
>>> to support some systems now.  On a system without HMAT/CDAT, it's
>>> possible to calculate abstract distance from ACPI SLIT, although this is
>>> quite limited.  I'm not sure whether all systems will provide read/write
>>> bandwith/latency data for all memory devices.
>>>
>>> HMAT and CDAT or some other mechanisms may provide the read/write
>>> bandwidth/latency data to be used to calculate abstract distance.  For
>>> them, we can provide a shared implementation in mm/memory-tiers.c to map
>>> from read/write bandwith/latency to the abstract distance.  Can this
>>> solve your concerns about the consistency among algorithms?  If so, we
>>> can do that when we add the second algorithm that needs that.
>>
>> I guess it would address my concerns if we did that now. I don't see why
>> we need to wait for a second implementation for that though - the whole
>> series seems to be built around adding a framework for supporting
>> multiple algorithms even though only one exists. So I think we should
>> support that fully, or simplfy the whole thing and just assume the only
>> thing that exists is HMAT and get rid of the general interface until a
>> second algorithm comes along.
>
> We will need a general interface even for one algorithm implementation.
> Because it's not good to make a dax subsystem driver (dax/kmem) to
> depend on a ACPI subsystem driver (acpi/hmat).  We need some general
> interface at subsystem level (memory tier here) between them.

I don't understand this argument. For a single algorithm it would be
simpler to just define acpi_hmat_calculate_adistance() and a static
inline version of it that returns -ENOENT when !CONFIG_ACPI than adding
a layer of indirection through notifier blocks. That breaks any
dependency on ACPI and there's plenty of precedent for this approach in
the kernel already.

Thanks,
Alistar.

> Best Regards,
> Huang, Ying

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