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