From: Alistair Popple <apopple@nvidia.com>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
linux-cxl@vger.kernel.org, nvdimm@lists.linux.dev,
linux-acpi@vger.kernel.org,
"Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
Wei Xu <weixugc@google.com>,
Dan Williams <dan.j.williams@intel.com>,
Dave Hansen <dave.hansen@intel.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Johannes Weiner <hannes@cmpxchg.org>,
Jonathan Cameron <Jonathan.Cameron@huawei.com>,
Michal Hocko <mhocko@kernel.org>, Yang Shi <shy828301@gmail.com>,
Rafael J Wysocki <rafael.j.wysocki@intel.com>,
Dave Jiang <dave.jiang@intel.com>
Subject: Re: [PATCH RESEND 1/4] memory tiering: add abstract distance calculation algorithms management
Date: Tue, 22 Aug 2023 17:11:34 +1000 [thread overview]
Message-ID: <87wmxnlfer.fsf@nvdebian.thelocal> (raw)
In-Reply-To: <875y57dhar.fsf@yhuang6-desk2.ccr.corp.intel.com>
"Huang, Ying" <ying.huang@intel.com> writes:
> Alistair Popple <apopple@nvidia.com> writes:
>
>> "Huang, Ying" <ying.huang@intel.com> writes:
>>
>>> Alistair Popple <apopple@nvidia.com> writes:
>>>
>>>> "Huang, Ying" <ying.huang@intel.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@nvidia.com> writes:
>>>>>
>>>>>> "Huang, Ying" <ying.huang@intel.com> writes:
>>>>>>
>>>>>>> Alistair Popple <apopple@nvidia.com> writes:
>>>>>>>
>>>>>>>> "Huang, Ying" <ying.huang@intel.com> writes:
>>>>>>>>
>>>>>>>>> Alistair Popple <apopple@nvidia.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.
>
> 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.
Thanks, I wasn't really thinking of HMAT as an ACPI driver. I understand
where you're coming from but I really don't see the problem with using a
static inline. It doesn't create dependencies (you could still use
dax/kmem without ACPI) and results in smaller and easier to follow code.
IMHO it's far more obvious that a call to acpi_hmat_calcaulte_adist()
returns either a default if ACPI HMAT isn't configured or a calculated
value than it is to figure out what notifiers may or may not be
registered at runtime and what priority they may be called in from
mt_calc_adistance().
It appears you think that is a bad design, but I don't understand
why. What does this approach give us that a simpler approach wouldn't?
next prev parent reply other threads:[~2023-08-22 7:12 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-21 1:29 [PATCH RESEND 0/4] memory tiering: calculate abstract distance based on ACPI HMAT Huang Ying
2023-07-21 1:29 ` [PATCH RESEND 1/4] memory tiering: add abstract distance calculation algorithms management Huang Ying
2023-07-25 2:13 ` Alistair Popple
2023-07-25 3:14 ` Huang, Ying
2023-07-25 8:26 ` Alistair Popple
2023-07-26 7:33 ` Huang, Ying
2023-07-27 3:42 ` Alistair Popple
2023-07-27 4:02 ` Huang, Ying
2023-07-27 4:07 ` Alistair Popple
2023-07-27 5:41 ` Huang, Ying
2023-07-28 1:20 ` Alistair Popple
2023-08-11 3:51 ` Huang, Ying
2023-08-21 11:26 ` Alistair Popple
2023-08-21 22:50 ` Huang, Ying
2023-08-21 23:52 ` Alistair Popple
2023-08-22 0:58 ` Huang, Ying
2023-08-22 7:11 ` Alistair Popple [this message]
2023-08-23 5:56 ` Huang, Ying
2023-08-25 5:41 ` Alistair Popple
2023-07-21 1:29 ` [PATCH RESEND 2/4] acpi, hmat: refactor hmat_register_target_initiators() Huang Ying
2023-07-25 2:44 ` Alistair Popple
2023-08-07 16:55 ` Jonathan Cameron
2023-08-11 1:13 ` Huang, Ying
2023-07-21 1:29 ` [PATCH RESEND 3/4] acpi, hmat: calculate abstract distance with HMAT Huang Ying
2023-07-25 2:45 ` Alistair Popple
2023-07-25 6:47 ` Huang, Ying
2023-08-21 11:53 ` Alistair Popple
2023-08-21 23:28 ` Huang, Ying
2023-07-21 1:29 ` [PATCH RESEND 4/4] dax, kmem: calculate abstract distance with general interface Huang Ying
2023-07-25 3:11 ` Alistair Popple
2023-07-25 7:02 ` Huang, Ying
2023-08-21 12:03 ` Alistair Popple
2023-08-21 23:33 ` Huang, Ying
2023-08-22 7:36 ` Alistair Popple
2023-08-23 2:13 ` Huang, Ying
2023-08-25 6:00 ` Alistair Popple
2023-07-21 4:15 ` [PATCH RESEND 0/4] memory tiering: calculate abstract distance based on ACPI HMAT Alistair Popple
2023-07-24 17:58 ` Andrew Morton
2023-08-01 2:35 ` Bharata B Rao
2023-08-11 6:26 ` Huang, Ying
2023-08-11 7:49 ` Bharata B Rao
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