Date:	Sat, 25 Apr 2015 04:55:09 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, jglisse@...hat.com, mgorman@...e.de,
	aarcange@...hat.com, airlied@...hat.com,
	aneesh.kumar@...ux.vnet.ibm.com,
	Cameron Buschardt <cabuschardt@...dia.com>,
	Mark Hairgrove <mhairgrove@...dia.com>,
	Geoffrey Gerfin <ggerfin@...dia.com>,
	John McKenna <jmckenna@...dia.com>, akpm@...ux-foundation.org
Subject: Re: Interacting with coherent memory on external devices

On Sat, Apr 25, 2015 at 01:32:39PM +1000, Benjamin Herrenschmidt wrote:
> On Fri, 2015-04-24 at 22:32 -0400, Rik van Riel wrote:
> > >       The result would be that the kernel would allocate only
> > migratable
> > >       pages within the CCAD device's memory, and even then only if
> > >       memory was otherwise exhausted.
> > 
> > Does it make sense to allocate the device's page tables in memory
> > belonging to the device?
> > 
> > Is this a necessary thing with some devices? Jerome's HMM comes
> > to mind...
> 
> In our case, the device's MMU shares the host page tables (which is why
> we can't use HMM, ie we can't have a page with different permissions on
> CPU vs. device which HMM does).
> 
> However the device has a pretty fast path to system memory, the best
> thing we can do is pin the workload to the same chip the device is
> connected to so those page tables arent' too far away.

And another update, diffs then full document.  Among other things, this
version explicitly calls out the goal of gaining substantial performance
without changing user applications, which should hopefully help.

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/DeviceMem.txt b/DeviceMem.txt
index 15d0a8b5d360..3de70c4b9922 100644
--- a/DeviceMem.txt
+++ b/DeviceMem.txt
@@ -40,10 +40,13 @@
 	workloads will have less-predictable access patterns, and these
 	workloads can benefit from automatic migration of data between
 	device memory and system memory as access patterns change.
-	Furthermore, some devices will provide special hardware that
-	collects access statistics that can be used to determine whether
-	or not a given page of memory should be migrated, and if so,
-	to where.
+	In this latter case, the goal is not optimal performance,
+	but rather a significant increase in performance compared to
+	what the CPUs alone can provide without needing to recompile
+	any of the applications making up the workload.  Furthermore,
+	some devices will provide special hardware that collects access
+	statistics that can be used to determine whether or not a given
+	page of memory should be migrated, and if so, to where.
 
 	The purpose of this document is to explore how this access
 	and migration can be provided for within the Linux kernel.
@@ -146,6 +149,32 @@ REQUIREMENTS
 		required for low-latency applications that are sensitive
 		to OS jitter.
 
+	6.	It must be possible to cause an application to use a
+		CCAD device simply by switching dynamically linked
+		libraries, but without recompiling that application.
+		This implies the following requirements:
+
+		a.	Address spaces must be synchronized for a given
+			application on the CPUs and the CCAD.  In other
+			words, a given virtual address must access the same
+			physical memory from the CCAD device and from
+			the CPUs.
+
+		b.	Code running on the CCAD device must be able to
+			access the running application's memory,
+			regardless of how that memory was allocated,
+			including statically allocated at compile time.
+
+		c.	Use of the CCAD device must not interfere with
+			memory allocations that are never used by the
+			CCAD device.  For example, if a CCAD device
+			has 16GB of memory, that should not prevent an
+			application using that device from allocating
+			more than 16GB of memory.  For another example,
+			memory that is never accessed by a given CCAD
+			device should preferably remain outside of that
+			CCAD device's memory.
+
 
 POTENTIAL IDEAS
 
@@ -178,12 +207,11 @@ POTENTIAL IDEAS
 		physical address ranges of normal system memory would
 		be interleaved with those of device memory.
 
-		This would also require some sort of
-		migration infrastructure to be added, as autonuma would
-		not apply.  However, this approach has the advantage
-		of preventing allocations in these regions, at least
-		unless those allocations have been explicitly flagged
-		to go there.
+		This would also require some sort of migration
+		infrastructure to be added, as autonuma would not apply.
+		However, this approach has the advantage of preventing
+		allocations in these regions, at least unless those
+		allocations have been explicitly flagged to go there.
 
 	4.	Your idea here!
 
@@ -274,21 +302,30 @@ MEMORY ZONE
 DAX
 
 	DAX is a mechanism for providing direct-memory access to
-	high-speed non-volatile (AKA "persistent") memory.  Good
-	introductions to DAX may be found in the following LWN
-	articles:
+	special memory, for example, to high-speed non-volatile (AKA
+	"persistent") memory.  A number of current use cases for DAX
+	put filesystems on top of DAX.	Good introductions to DAX may
+	be found in the following LWN articles:
 
 		https://lwn.net/Articles/591779/
 		https://lwn.net/Articles/610174/
+		https://lwn.net/Articles/640113/
+
+	DAX is now in mainline, see for example fs/dax.c.
+
+	One important CCAD use case allows an unmodified legacy
+	application to pass some memory to a CCAD device, no matter how
+	this memory was allocated, while leaving other memory in system
+	memory, even if this other memory was allocated in exactly
+	the same way.  The intent to use migration to move the memory
+	as required.  DAX does not seem to help much with this use case.
 
-	DAX provides filesystem-level access to persistent memory.
-	One important CCAD use case is allowing a legacy application
-	to pass memory from malloc() to a CCAD device, and having
-	the allocated memory migrate as needed.  DAX does not seem to
-	support this use case.
+	There has been some discussion of associating struct page
+	structures, which might (or might not) make DAX a better fit
+	for CCAD.
 
 
 ACKNOWLEDGMENTS
 
-	Updates to this document include feedback from Christoph Lameter
-	and Jerome Glisse.
+	Updates to this document include feedback from Christoph Lameter,
+	Jerome Glisse, Rik van Riel, Austin S Hemmelgarn, and Oded Gabbay.

------------------------------------------------------------------------

           COHERENT ON-DEVICE MEMORY: ACCESS AND MIGRATION
                         Ben Herrenschmidt
                   (As told to Paul E. McKenney)

	Special-purpose hardware becoming more prevalent, and some of this
	hardware allows for tight interaction with CPU-based processing.
	For example, IBM's coherent accelerator processor interface
	(CAPI) will allow this sort of device to be constructed,
	and it is likely that GPGPUs will need similar capabilities.
	(See http://www-304.ibm.com/webapp/set2/sas/f/capi/home.html for a
	high-level description of CAPI.)  Let's call these cache-coherent
	accelerator devices (CCAD for short, which should at least
	motivate someone to come up with something better).

	This document covers devices with the following properties:

	1.	The device is cache-coherent, in other words, the device's
		memory has all the characteristics of system memory from
		the viewpoint of CPUs and other devices accessing it.

	2.	The device provides local memory that it has high-bandwidth
		low-latency access to, but the device can also access
		normal system memory.

	3.	The device shares system page tables, so that it can
		transparently access userspace virtual memory, regardless
		of whether this virtual memory maps to normal system
		memory or to memory local to the device.

	Although such a device will provide CPU's with cache-coherent
	access to on-device memory, the resulting memory latency is
	expected to be slower than the normal memory that is tightly
	coupled to the CPUs.  Nevertheless, data that is only occasionally
	accessed by CPUs should be stored in the device's memory.
	On the other hand, data that is accessed rarely by the device but
	frequently by the CPUs should be stored in normal system memory.

	Of course, some workloads will have predictable access patterns
	that allow data to be optimally placed up front.  However, other
	workloads will have less-predictable access patterns, and these
	workloads can benefit from automatic migration of data between
	device memory and system memory as access patterns change.
	In this latter case, the goal is not optimal performance,
	but rather a significant increase in performance compared to
	what the CPUs alone can provide without needing to recompile
	any of the applications making up the workload.  Furthermore,
	some devices will provide special hardware that collects access
	statistics that can be used to determine whether or not a given
	page of memory should be migrated, and if so, to where.

	The purpose of this document is to explore how this access
	and migration can be provided for within the Linux kernel.


USE CASES

	o	Multiple transformations without requiring multiple
		memory transfers for throughput-oriented applications.
		For example, suppose the device supports both compression
		and encryption algorithms, but that significant CPU
		work is required to generate the data to be compressed
		and encrypted.	Suppose also that the application uses
		a library to do the compression and encryption, and
		that this application needs to run correctly, without
		rebuilding, on systems with the device and also on systems
		without the device.  In addition, the application operates
		on data mapped from files, data in normal data/bss memory,
		and data in heap memory from malloc().

		In this case, it would be beneficial to have the memory
		automatically migrate to and from device memory.
		Note that the device-specific library functions could
		reasonably initiate the migration before starting their
		work, but could not know whether or not to migrate the
		data back upon completion.

	o	A special-purpose globally hand-optimized application
		wishes to use the device, from Christoph Lameter.

		In this case, the application will get the absolute
		best performance by manually controlling allocation
		and migration decisions.  This use case is probably
		not helped much by this proposal.

		However, an application including a special-purpose
		hand-optimized core and less-intense ancillary processing
		could well benefit.

	o	GPGPU matrix operations, from Jerome Glisse.
		https://lkml.org/lkml/2015/4/21/898

		Suppose that you have an application that uses a
		scientific library to do matrix computations, and that
		this application simply calls malloc() and give the
		resulting pointer to the library function.  If the GPGPU
		has coherent access to system memory (and vice versa),
		it would help performance and application compatibility
		to be able to transparently migrate the malloc()ed
		memory to and from the GPGPU's memory without requiring
		changes to the application.

	o	(More here for CAPI.)


REQUIREMENTS

	1.	It should be possible to remove a given CCAD device
		from service, for example, to reset it, to download
		updated firmware, or to change its functionality.
		This results in the following additional requirements:

		a.	It should be possible to migrate all data away
			from the device's memory at any time.

		b.	Normal memory allocation should avoid using the
			device's memory, as this would interfere
			with the needed migration.  It may nevertheless
			be desirable to use the device's memory
			if system memory is exhausted, however, in some
			cases, even this "emergency" use is best avoided.
			In fact, a good solution will provide some means
			for avoiding this for those cases where it is
			necessary to evacuate memory when offlining the
			device.

	2.	Memory can be either explicitly or implicitly allocated
		from the CCAD device's memory.	(Both usermode and kernel
		allocation required.)

		Please note that implicit allocation will need to be
		avoided in a number of use cases.  The reason for this
		is that random kernel allocations might be pinned into
		memory, which could conflict with requirement (1) above,
		and might furthermore fragment the device's memory.

	3.	The device's memory is treated like normal system
		memory by the Linux kernel, for example, each page has a
		"struct page" associate with it.  (In contrast, the
		traditional approach has used special-purpose OS mechanisms
		to manage the device's memory, and this memory was treated
		as MMIO space by the kernel.)

	4.	The system's normal tuning mechanism may be used to
		tune allocation locality, migration, and so on, as
		required to match performance and functional requirements.

	5.	It must be possible to configure a system containing
		a CCAD device so that it does no migration, as will be
		required for low-latency applications that are sensitive
		to OS jitter.

	6.	It must be possible to cause an application to use a
		CCAD device simply by switching dynamically linked
		libraries, but without recompiling that application.
		This implies the following requirements:

		a.	Address spaces must be synchronized for a given
			application on the CPUs and the CCAD.  In other
			words, a given virtual address must access the same
			physical memory from the CCAD device and from
			the CPUs.

		b.	Code running on the CCAD device must be able to
			access the running application's memory,
			regardless of how that memory was allocated,
			including statically allocated at compile time.

		c.	Use of the CCAD device must not interfere with
			memory allocations that are never used by the
			CCAD device.  For example, if a CCAD device
			has 16GB of memory, that should not prevent an
			application using that device from allocating
			more than 16GB of memory.  For another example,
			memory that is never accessed by a given CCAD
			device should preferably remain outside of that
			CCAD device's memory.


POTENTIAL IDEAS

	It is only reasonable to ask whether CCAD devices can simply
	use the HMM patch that has recently been proposed to allow
	migration between system and device memory via page faults.
	Although this works well for devices whose local MMU can contain
	mappings different from that of the system MMU, the HMM patch
	is still working with MMIO space that gets special treatment.
	The HMM patch does not (yet) provide the full transparency that
	would allow the device memory to be treated in the same way as
	system memory.	Something more is therefore required, for example,
	one or more of the following:

	1.	Model the CCAD device's memory as a memory-only NUMA node
		with a very large distance metric.  This allows use of
		the existing mechanisms for choosing where to satisfy
		explicit allocations and where to target migrations.
		
	2.	Cover the memory with a CMA to prevent non-migratable
		pinned data from being placed in the CCAD device's memory.
		It would also permit the driver to perform dedicated
		physically contiguous allocations as needed.

	3.	Add a new ZONE_EXTERNAL zone for all CCAD-like devices.
		Note that this would likely require support for
		discontinuous zones in order to support large NUMA
		systems, in which each node has a single block of the
		overall physical address space.  In such systems, the
		physical address ranges of normal system memory would
		be interleaved with those of device memory.

		This would also require some sort of migration
		infrastructure to be added, as autonuma would not apply.
		However, this approach has the advantage of preventing
		allocations in these regions, at least unless those
		allocations have been explicitly flagged to go there.

	4.	Your idea here!


The following sections cover AutoNUMA, use of memory zones, and DAX.


AUTONUMA

	The Linux kernel's autonuma facility supports migrating both
	memory and processes to promote NUMA memory locality.  It was
	accepted into 3.13 and is available in RHEL 7.0 and SLES 12.
	It is enabled by the Kconfig variable CONFIG_NUMA_BALANCING.

	This approach uses a kernel thread "knuma_scand" that periodically
	marks pages inaccessible.  The page-fault handler notes any
	mismatches between the NUMA node that the process is running on
	and the NUMA node on which the page resides.

	http://lwn.net/Articles/488709/
	https://www.kernel.org/pub/linux/kernel/people/andrea/autonuma/autonuma_bench-20120530.pdf

	It will be necessary to set up the CCAD device's memory as
	a very distant NUMA node, and the architecture-specific
	__numa_distance() function can be used for this purpose.
	There is a RECLAIM_DISTANCE macro that can be set by the
	architecture to prevent reclaiming from nodes that are too
	far away.  Some experimentation would be required to determine
	the combination of values for the various distance macros.

	This approach needs some way to pull in data from the hardware
	on access patterns.  Aneesh Kk Veetil is prototyping an approach
	based on Power 8 hardware counters.  This data will need to be
	plugged into the migration algorithm, which is currently based
	on collecting information from page faults.

	Finally, the contiguous memory allocator (CMA, see
	http://lwn.net/Articles/486301/) is needed in order to prevent
	the kernel from placing non-migratable allocations in the CCAD
	device's memory.  This would need to be of type MIGRATE_CMA to
	ensure that all memory taken from that range be migratable.

	The result would be that the kernel would allocate only migratable
	pages within the CCAD device's memory, and even then only if
	memory was otherwise exhausted.  Normal CONFIG_NUMA_BALANCING
	migration could be brought to bear, possibly enhanced with
	information from hardware counters.  One remaining issue is that
	there is no way to absolutely prevent random kernel subsystems
	from allocating the CCAD device's memory, which could cause
	failures should the device need to reset itself, in which case
	the memory would be temporarily inaccessible -- which could be
	a fatal surprise to that kernel subsystem.

	Jerome Glisse suggests that usermode hints are quite important,
	and perhaps should replace any AutoNUMA measurements.


MEMORY ZONE

	One way to avoid the problem of random kernel subsystems using
	the CAPI device's memory is to create a new memory zone for
	this purpose.  This would add something like ZONE_DEVMEM to the
	current set that includes ZONE_DMA, ZONE_NORMAL, and ZONE_MOVABLE.
	Currently, there are a maximum of four zones, so this limit must
	either be increased or kernels built with ZONE_DEVMEM must avoid
	having more than one of ZONE_DMA, ZONE_DMA32, and ZONE_HIGHMEM.

	This approach requires that migration be implemented on the side,
	as the CONFIG_NUMA_BALANCING will not help here (unless I am
	missing something).  One advantage of this situation is that
	hardware locality measurements could be incorporated from the
	beginning.  Another advantage is that random kernel subsystems
	and user programs would not get CAPI device memory unless they
	explicitly requested it.

	Code would be needed at boot time to place the CAPI device
	memory into ZONE_DEVMEM, perhaps involving changes to
	mem_init() and paging_init().

	In addition, an appropriate GFP_DEVMEM would be needed, along
	with code in various paths to handle it appropriately.

	Also, because large NUMA systems will sometimes interleave the
	addresses of blocks of physical memory and device memory,
	support for discontiguous interleaved zones will be required.


DAX

	DAX is a mechanism for providing direct-memory access to
	special memory, for example, to high-speed non-volatile (AKA
	"persistent") memory.  A number of current use cases for DAX
	put filesystems on top of DAX.	Good introductions to DAX may
	be found in the following LWN articles:

		https://lwn.net/Articles/591779/
		https://lwn.net/Articles/610174/
		https://lwn.net/Articles/640113/

	DAX is now in mainline, see for example fs/dax.c.

	One important CCAD use case allows an unmodified legacy
	application to pass some memory to a CCAD device, no matter how
	this memory was allocated, while leaving other memory in system
	memory, even if this other memory was allocated in exactly
	the same way.  The intent to use migration to move the memory
	as required.  DAX does not seem to help much with this use case.

	There has been some discussion of associating struct page
	structures, which might (or might not) make DAX a better fit
	for CCAD.


ACKNOWLEDGMENTS

	Updates to this document include feedback from Christoph Lameter,
	Jerome Glisse, Rik van Riel, Austin S Hemmelgarn, and Oded Gabbay.

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