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: >>>> >>>>> "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. > > 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? Think about all these again. Finally I admit you are right. The general interface is better mainly if there are multiple implementations of the interface. In this series, we provide just one implementation: HMAT. And, the second one: CDAT will be implemented soon. And, CDAT will use the same method to translate from read/write bandwidth/latency to adistance. So, I suggest to: - Keep the general interface (and notifier chain), for HMAT and soon available CDAT - Move the code to translate from read/write bandwidth/latency to adistance to memory-tiers.c. Which is used by HMAT now and will be used by CDAT soon. And it can be used by other drivers. What do you think about that? -- Best Regards, Huang, Ying
