387 lines
18 KiB
ReStructuredText
387 lines
18 KiB
ReStructuredText
.. hmm:
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=====================================
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Heterogeneous Memory Management (HMM)
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=====================================
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Provide infrastructure and helpers to integrate non-conventional memory (device
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memory like GPU on board memory) into regular kernel path, with the cornerstone
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of this being specialized struct page for such memory (see sections 5 to 7 of
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this document).
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HMM also provides optional helpers for SVM (Share Virtual Memory), i.e.,
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allowing a device to transparently access program address coherently with
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the CPU meaning that any valid pointer on the CPU is also a valid pointer
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for the device. This is becoming mandatory to simplify the use of advanced
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heterogeneous computing where GPU, DSP, or FPGA are used to perform various
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computations on behalf of a process.
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This document is divided as follows: in the first section I expose the problems
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related to using device specific memory allocators. In the second section, I
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expose the hardware limitations that are inherent to many platforms. The third
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section gives an overview of the HMM design. The fourth section explains how
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CPU page-table mirroring works and the purpose of HMM in this context. The
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fifth section deals with how device memory is represented inside the kernel.
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Finally, the last section presents a new migration helper that allows lever-
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aging the device DMA engine.
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.. contents:: :local:
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Problems of using a device specific memory allocator
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====================================================
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Devices with a large amount of on board memory (several gigabytes) like GPUs
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have historically managed their memory through dedicated driver specific APIs.
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This creates a disconnect between memory allocated and managed by a device
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driver and regular application memory (private anonymous, shared memory, or
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regular file backed memory). From here on I will refer to this aspect as split
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address space. I use shared address space to refer to the opposite situation:
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i.e., one in which any application memory region can be used by a device
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transparently.
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Split address space happens because device can only access memory allocated
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through device specific API. This implies that all memory objects in a program
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are not equal from the device point of view which complicates large programs
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that rely on a wide set of libraries.
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Concretely this means that code that wants to leverage devices like GPUs needs
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to copy object between generically allocated memory (malloc, mmap private, mmap
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share) and memory allocated through the device driver API (this still ends up
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with an mmap but of the device file).
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For flat data sets (array, grid, image, ...) this isn't too hard to achieve but
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complex data sets (list, tree, ...) are hard to get right. Duplicating a
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complex data set needs to re-map all the pointer relations between each of its
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elements. This is error prone and program gets harder to debug because of the
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duplicate data set and addresses.
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Split address space also means that libraries cannot transparently use data
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they are getting from the core program or another library and thus each library
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might have to duplicate its input data set using the device specific memory
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allocator. Large projects suffer from this and waste resources because of the
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various memory copies.
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Duplicating each library API to accept as input or output memory allocated by
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each device specific allocator is not a viable option. It would lead to a
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combinatorial explosion in the library entry points.
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Finally, with the advance of high level language constructs (in C++ but in
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other languages too) it is now possible for the compiler to leverage GPUs and
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other devices without programmer knowledge. Some compiler identified patterns
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are only do-able with a shared address space. It is also more reasonable to use
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a shared address space for all other patterns.
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I/O bus, device memory characteristics
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======================================
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I/O buses cripple shared address spaces due to a few limitations. Most I/O
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buses only allow basic memory access from device to main memory; even cache
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coherency is often optional. Access to device memory from CPU is even more
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limited. More often than not, it is not cache coherent.
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If we only consider the PCIE bus, then a device can access main memory (often
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through an IOMMU) and be cache coherent with the CPUs. However, it only allows
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a limited set of atomic operations from device on main memory. This is worse
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in the other direction: the CPU can only access a limited range of the device
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memory and cannot perform atomic operations on it. Thus device memory cannot
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be considered the same as regular memory from the kernel point of view.
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Another crippling factor is the limited bandwidth (~32GBytes/s with PCIE 4.0
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and 16 lanes). This is 33 times less than the fastest GPU memory (1 TBytes/s).
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The final limitation is latency. Access to main memory from the device has an
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order of magnitude higher latency than when the device accesses its own memory.
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Some platforms are developing new I/O buses or additions/modifications to PCIE
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to address some of these limitations (OpenCAPI, CCIX). They mainly allow two-
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way cache coherency between CPU and device and allow all atomic operations the
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architecture supports. Sadly, not all platforms are following this trend and
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some major architectures are left without hardware solutions to these problems.
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So for shared address space to make sense, not only must we allow devices to
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access any memory but we must also permit any memory to be migrated to device
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memory while device is using it (blocking CPU access while it happens).
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Shared address space and migration
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==================================
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HMM intends to provide two main features. First one is to share the address
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space by duplicating the CPU page table in the device page table so the same
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address points to the same physical memory for any valid main memory address in
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the process address space.
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To achieve this, HMM offers a set of helpers to populate the device page table
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while keeping track of CPU page table updates. Device page table updates are
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not as easy as CPU page table updates. To update the device page table, you must
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allocate a buffer (or use a pool of pre-allocated buffers) and write GPU
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specific commands in it to perform the update (unmap, cache invalidations, and
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flush, ...). This cannot be done through common code for all devices. Hence
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why HMM provides helpers to factor out everything that can be while leaving the
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hardware specific details to the device driver.
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The second mechanism HMM provides is a new kind of ZONE_DEVICE memory that
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allows allocating a struct page for each page of the device memory. Those pages
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are special because the CPU cannot map them. However, they allow migrating
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main memory to device memory using existing migration mechanisms and everything
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looks like a page is swapped out to disk from the CPU point of view. Using a
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struct page gives the easiest and cleanest integration with existing mm mech-
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anisms. Here again, HMM only provides helpers, first to hotplug new ZONE_DEVICE
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memory for the device memory and second to perform migration. Policy decisions
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of what and when to migrate things is left to the device driver.
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Note that any CPU access to a device page triggers a page fault and a migration
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back to main memory. For example, when a page backing a given CPU address A is
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migrated from a main memory page to a device page, then any CPU access to
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address A triggers a page fault and initiates a migration back to main memory.
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With these two features, HMM not only allows a device to mirror process address
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space and keeping both CPU and device page table synchronized, but also lever-
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ages device memory by migrating the part of the data set that is actively being
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used by the device.
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Address space mirroring implementation and API
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==============================================
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Address space mirroring's main objective is to allow duplication of a range of
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CPU page table into a device page table; HMM helps keep both synchronized. A
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device driver that wants to mirror a process address space must start with the
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registration of an hmm_mirror struct::
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int hmm_mirror_register(struct hmm_mirror *mirror,
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struct mm_struct *mm);
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int hmm_mirror_register_locked(struct hmm_mirror *mirror,
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struct mm_struct *mm);
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The locked variant is to be used when the driver is already holding mmap_sem
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of the mm in write mode. The mirror struct has a set of callbacks that are used
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to propagate CPU page tables::
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struct hmm_mirror_ops {
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/* sync_cpu_device_pagetables() - synchronize page tables
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*
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* @mirror: pointer to struct hmm_mirror
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* @update_type: type of update that occurred to the CPU page table
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* @start: virtual start address of the range to update
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* @end: virtual end address of the range to update
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*
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* This callback ultimately originates from mmu_notifiers when the CPU
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* page table is updated. The device driver must update its page table
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* in response to this callback. The update argument tells what action
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* to perform.
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*
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* The device driver must not return from this callback until the device
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* page tables are completely updated (TLBs flushed, etc); this is a
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* synchronous call.
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*/
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void (*update)(struct hmm_mirror *mirror,
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enum hmm_update action,
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unsigned long start,
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unsigned long end);
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};
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The device driver must perform the update action to the range (mark range
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read only, or fully unmap, ...). The device must be done with the update before
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the driver callback returns.
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When the device driver wants to populate a range of virtual addresses, it can
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use either::
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int hmm_vma_get_pfns(struct vm_area_struct *vma,
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struct hmm_range *range,
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unsigned long start,
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unsigned long end,
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hmm_pfn_t *pfns);
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int hmm_vma_fault(struct vm_area_struct *vma,
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struct hmm_range *range,
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unsigned long start,
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unsigned long end,
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hmm_pfn_t *pfns,
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bool write,
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bool block);
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The first one (hmm_vma_get_pfns()) will only fetch present CPU page table
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entries and will not trigger a page fault on missing or non-present entries.
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The second one does trigger a page fault on missing or read-only entry if the
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write parameter is true. Page faults use the generic mm page fault code path
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just like a CPU page fault.
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Both functions copy CPU page table entries into their pfns array argument. Each
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entry in that array corresponds to an address in the virtual range. HMM
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provides a set of flags to help the driver identify special CPU page table
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entries.
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Locking with the update() callback is the most important aspect the driver must
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respect in order to keep things properly synchronized. The usage pattern is::
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int driver_populate_range(...)
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{
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struct hmm_range range;
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...
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again:
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ret = hmm_vma_get_pfns(vma, &range, start, end, pfns);
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if (ret)
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return ret;
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take_lock(driver->update);
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if (!hmm_vma_range_done(vma, &range)) {
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release_lock(driver->update);
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goto again;
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}
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// Use pfns array content to update device page table
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release_lock(driver->update);
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return 0;
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}
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The driver->update lock is the same lock that the driver takes inside its
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update() callback. That lock must be held before hmm_vma_range_done() to avoid
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any race with a concurrent CPU page table update.
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HMM implements all this on top of the mmu_notifier API because we wanted a
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simpler API and also to be able to perform optimizations latter on like doing
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concurrent device updates in multi-devices scenario.
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HMM also serves as an impedance mismatch between how CPU page table updates
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are done (by CPU write to the page table and TLB flushes) and how devices
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update their own page table. Device updates are a multi-step process. First,
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appropriate commands are written to a buffer, then this buffer is scheduled for
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execution on the device. It is only once the device has executed commands in
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the buffer that the update is done. Creating and scheduling the update command
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buffer can happen concurrently for multiple devices. Waiting for each device to
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report commands as executed is serialized (there is no point in doing this
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concurrently).
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Represent and manage device memory from core kernel point of view
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=================================================================
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Several different designs were tried to support device memory. First one used
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a device specific data structure to keep information about migrated memory and
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HMM hooked itself in various places of mm code to handle any access to
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addresses that were backed by device memory. It turns out that this ended up
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replicating most of the fields of struct page and also needed many kernel code
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paths to be updated to understand this new kind of memory.
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Most kernel code paths never try to access the memory behind a page
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but only care about struct page contents. Because of this, HMM switched to
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directly using struct page for device memory which left most kernel code paths
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unaware of the difference. We only need to make sure that no one ever tries to
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map those pages from the CPU side.
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HMM provides a set of helpers to register and hotplug device memory as a new
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region needing a struct page. This is offered through a very simple API::
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struct hmm_devmem *hmm_devmem_add(const struct hmm_devmem_ops *ops,
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struct device *device,
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unsigned long size);
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void hmm_devmem_remove(struct hmm_devmem *devmem);
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The hmm_devmem_ops is where most of the important things are::
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struct hmm_devmem_ops {
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void (*free)(struct hmm_devmem *devmem, struct page *page);
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int (*fault)(struct hmm_devmem *devmem,
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struct vm_area_struct *vma,
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unsigned long addr,
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struct page *page,
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unsigned flags,
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pmd_t *pmdp);
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};
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The first callback (free()) happens when the last reference on a device page is
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dropped. This means the device page is now free and no longer used by anyone.
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The second callback happens whenever the CPU tries to access a device page
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which it cannot do. This second callback must trigger a migration back to
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system memory.
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Migration to and from device memory
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===================================
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Because the CPU cannot access device memory, migration must use the device DMA
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engine to perform copy from and to device memory. For this we need a new
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migration helper::
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int migrate_vma(const struct migrate_vma_ops *ops,
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struct vm_area_struct *vma,
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unsigned long mentries,
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unsigned long start,
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unsigned long end,
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unsigned long *src,
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unsigned long *dst,
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void *private);
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Unlike other migration functions it works on a range of virtual address, there
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are two reasons for that. First, device DMA copy has a high setup overhead cost
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and thus batching multiple pages is needed as otherwise the migration overhead
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makes the whole exercise pointless. The second reason is because the
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migration might be for a range of addresses the device is actively accessing.
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The migrate_vma_ops struct defines two callbacks. First one (alloc_and_copy())
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controls destination memory allocation and copy operation. Second one is there
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to allow the device driver to perform cleanup operations after migration::
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struct migrate_vma_ops {
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void (*alloc_and_copy)(struct vm_area_struct *vma,
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const unsigned long *src,
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unsigned long *dst,
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unsigned long start,
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unsigned long end,
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void *private);
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void (*finalize_and_map)(struct vm_area_struct *vma,
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const unsigned long *src,
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const unsigned long *dst,
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unsigned long start,
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unsigned long end,
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void *private);
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};
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It is important to stress that these migration helpers allow for holes in the
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virtual address range. Some pages in the range might not be migrated for all
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the usual reasons (page is pinned, page is locked, ...). This helper does not
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fail but just skips over those pages.
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The alloc_and_copy() might decide to not migrate all pages in the
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range (for reasons under the callback control). For those, the callback just
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has to leave the corresponding dst entry empty.
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Finally, the migration of the struct page might fail (for file backed page) for
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various reasons (failure to freeze reference, or update page cache, ...). If
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that happens, then the finalize_and_map() can catch any pages that were not
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migrated. Note those pages were still copied to a new page and thus we wasted
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bandwidth but this is considered as a rare event and a price that we are
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willing to pay to keep all the code simpler.
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Memory cgroup (memcg) and rss accounting
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========================================
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For now device memory is accounted as any regular page in rss counters (either
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anonymous if device page is used for anonymous, file if device page is used for
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file backed page or shmem if device page is used for shared memory). This is a
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deliberate choice to keep existing applications, that might start using device
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memory without knowing about it, running unimpacted.
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A drawback is that the OOM killer might kill an application using a lot of
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device memory and not a lot of regular system memory and thus not freeing much
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system memory. We want to gather more real world experience on how applications
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and system react under memory pressure in the presence of device memory before
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deciding to account device memory differently.
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Same decision was made for memory cgroup. Device memory pages are accounted
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against same memory cgroup a regular page would be accounted to. This does
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simplify migration to and from device memory. This also means that migration
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back from device memory to regular memory cannot fail because it would
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go above memory cgroup limit. We might revisit this choice latter on once we
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get more experience in how device memory is used and its impact on memory
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resource control.
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Note that device memory can never be pinned by device driver nor through GUP
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and thus such memory is always free upon process exit. Or when last reference
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is dropped in case of shared memory or file backed memory.
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