Re: [PATCH RESEND 1/4] memory tiering: add abstract distance calculation algorithms management

[Huang, Ying]

Alistair Popple <apopple@xxxxxxxxxx> writes:

> "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
>
>> Alistair Popple <apopple@xxxxxxxxxx> writes:
>>
>>> "Huang, Ying" <ying.huang@xxxxxxxxx> 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@xxxxxxxxxx> writes:
>>>>
>>>>> "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
>>>>>
>>>>>> Alistair Popple <apopple@xxxxxxxxxx> writes:
>>>>>>
>>>>>>> "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
>>>>>>>
>>>>>>>> Alistair Popple <apopple@xxxxxxxxxx> 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.

ACPI is a subsystem, so it's OK for dax/kmem to depends on CONFIG_ACPI.
But HMAT is a driver of ACPI subsystem (controlled via
CONFIG_ACPI_HMAT).  It's not good for a driver of DAX subsystem
(dax/kmem) to depend on a *driver* of ACPI subsystem.

Yes.  Technically, there's no hard wall to prevent this.  But I think
that a good design should make drivers depends on subsystems or drivers
of the same subsystem, NOT drivers of other subsystems.

--
Best Regards,
Huang, Ying