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![Michal Hocko](/assets/img/avatar_default.png)
Patch series "cleanup zonelists initialization", v1. This is aimed at cleaning up the zonelists initialization code we have but the primary motivation was bug report [2] which got resolved but the usage of stop_machine is just too ugly to live. Most patches are straightforward but 3 of them need a special consideration. Patch 1 removes zone ordered zonelists completely. I am CCing linux-api because this is a user visible change. As I argue in the patch description I do not think we have a strong usecase for it these days. I have kept sysctl in place and warn into the log if somebody tries to configure zone lists ordering. If somebody has a real usecase for it we can revert this patch but I do not expect anybody will actually notice runtime differences. This patch is not strictly needed for the rest but it made patch 6 easier to implement. Patch 7 removes stop_machine from build_all_zonelists without adding any special synchronization between iterators and updater which I _believe_ is acceptable as explained in the changelog. I hope I am not missing anything. Patch 8 then removes zonelists_mutex which is kind of ugly as well and not really needed AFAICS but a care should be taken when double checking my thinking. This patch (of 9): Supporting zone ordered zonelists costs us just a lot of code while the usefulness is arguable if existent at all. Mel has already made node ordering default on 64b systems. 32b systems are still using ZONELIST_ORDER_ZONE because it is considered better to fallback to a different NUMA node rather than consume precious lowmem zones. This argument is, however, weaken by the fact that the memory reclaim has been reworked to be node rather than zone oriented. This means that lowmem requests have to skip over all highmem pages on LRUs already and so zone ordering doesn't save the reclaim time much. So the only advantage of the zone ordering is under a light memory pressure when highmem requests do not ever hit into lowmem zones and the lowmem pressure doesn't need to reclaim. Considering that 32b NUMA systems are rather suboptimal already and it is generally advisable to use 64b kernel on such a HW I believe we should rather care about the code maintainability and just get rid of ZONELIST_ORDER_ZONE altogether. Keep systcl in place and warn if somebody tries to set zone ordering either from kernel command line or the sysctl. [mhocko@suse.com: reading vm.numa_zonelist_order will never terminate] Link: http://lkml.kernel.org/r/20170721143915.14161-2-mhocko@kernel.org Signed-off-by: Michal Hocko <mhocko@suse.com> Acked-by: Mel Gorman <mgorman@suse.de> Acked-by: Vlastimil Babka <vbabka@suse.cz> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Joonsoo Kim <js1304@gmail.com> Cc: Shaohua Li <shaohua.li@intel.com> Cc: Toshi Kani <toshi.kani@hpe.com> Cc: Abdul Haleem <abdhalee@linux.vnet.ibm.com> Cc: <linux-api@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
146 lines
8.5 KiB
Text
146 lines
8.5 KiB
Text
Started Nov 1999 by Kanoj Sarcar <kanoj@sgi.com>
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What is NUMA?
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This question can be answered from a couple of perspectives: the
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hardware view and the Linux software view.
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From the hardware perspective, a NUMA system is a computer platform that
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comprises multiple components or assemblies each of which may contain 0
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or more CPUs, local memory, and/or IO buses. For brevity and to
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disambiguate the hardware view of these physical components/assemblies
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from the software abstraction thereof, we'll call the components/assemblies
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'cells' in this document.
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Each of the 'cells' may be viewed as an SMP [symmetric multi-processor] subset
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of the system--although some components necessary for a stand-alone SMP system
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may not be populated on any given cell. The cells of the NUMA system are
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connected together with some sort of system interconnect--e.g., a crossbar or
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point-to-point link are common types of NUMA system interconnects. Both of
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these types of interconnects can be aggregated to create NUMA platforms with
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cells at multiple distances from other cells.
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For Linux, the NUMA platforms of interest are primarily what is known as Cache
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Coherent NUMA or ccNUMA systems. With ccNUMA systems, all memory is visible
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to and accessible from any CPU attached to any cell and cache coherency
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is handled in hardware by the processor caches and/or the system interconnect.
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Memory access time and effective memory bandwidth varies depending on how far
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away the cell containing the CPU or IO bus making the memory access is from the
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cell containing the target memory. For example, access to memory by CPUs
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attached to the same cell will experience faster access times and higher
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bandwidths than accesses to memory on other, remote cells. NUMA platforms
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can have cells at multiple remote distances from any given cell.
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Platform vendors don't build NUMA systems just to make software developers'
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lives interesting. Rather, this architecture is a means to provide scalable
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memory bandwidth. However, to achieve scalable memory bandwidth, system and
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application software must arrange for a large majority of the memory references
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[cache misses] to be to "local" memory--memory on the same cell, if any--or
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to the closest cell with memory.
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This leads to the Linux software view of a NUMA system:
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Linux divides the system's hardware resources into multiple software
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abstractions called "nodes". Linux maps the nodes onto the physical cells
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of the hardware platform, abstracting away some of the details for some
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architectures. As with physical cells, software nodes may contain 0 or more
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CPUs, memory and/or IO buses. And, again, memory accesses to memory on
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"closer" nodes--nodes that map to closer cells--will generally experience
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faster access times and higher effective bandwidth than accesses to more
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remote cells.
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For some architectures, such as x86, Linux will "hide" any node representing a
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physical cell that has no memory attached, and reassign any CPUs attached to
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that cell to a node representing a cell that does have memory. Thus, on
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these architectures, one cannot assume that all CPUs that Linux associates with
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a given node will see the same local memory access times and bandwidth.
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In addition, for some architectures, again x86 is an example, Linux supports
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the emulation of additional nodes. For NUMA emulation, linux will carve up
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the existing nodes--or the system memory for non-NUMA platforms--into multiple
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nodes. Each emulated node will manage a fraction of the underlying cells'
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physical memory. NUMA emluation is useful for testing NUMA kernel and
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application features on non-NUMA platforms, and as a sort of memory resource
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management mechanism when used together with cpusets.
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[see Documentation/cgroup-v1/cpusets.txt]
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For each node with memory, Linux constructs an independent memory management
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subsystem, complete with its own free page lists, in-use page lists, usage
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statistics and locks to mediate access. In addition, Linux constructs for
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each memory zone [one or more of DMA, DMA32, NORMAL, HIGH_MEMORY, MOVABLE],
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an ordered "zonelist". A zonelist specifies the zones/nodes to visit when a
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selected zone/node cannot satisfy the allocation request. This situation,
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when a zone has no available memory to satisfy a request, is called
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"overflow" or "fallback".
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Because some nodes contain multiple zones containing different types of
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memory, Linux must decide whether to order the zonelists such that allocations
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fall back to the same zone type on a different node, or to a different zone
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type on the same node. This is an important consideration because some zones,
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such as DMA or DMA32, represent relatively scarce resources. Linux chooses
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a default Node ordered zonelist. This means it tries to fallback to other zones
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from the same node before using remote nodes which are ordered by NUMA distance.
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By default, Linux will attempt to satisfy memory allocation requests from the
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node to which the CPU that executes the request is assigned. Specifically,
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Linux will attempt to allocate from the first node in the appropriate zonelist
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for the node where the request originates. This is called "local allocation."
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If the "local" node cannot satisfy the request, the kernel will examine other
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nodes' zones in the selected zonelist looking for the first zone in the list
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that can satisfy the request.
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Local allocation will tend to keep subsequent access to the allocated memory
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"local" to the underlying physical resources and off the system interconnect--
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as long as the task on whose behalf the kernel allocated some memory does not
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later migrate away from that memory. The Linux scheduler is aware of the
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NUMA topology of the platform--embodied in the "scheduling domains" data
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structures [see Documentation/scheduler/sched-domains.txt]--and the scheduler
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attempts to minimize task migration to distant scheduling domains. However,
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the scheduler does not take a task's NUMA footprint into account directly.
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Thus, under sufficient imbalance, tasks can migrate between nodes, remote
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from their initial node and kernel data structures.
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System administrators and application designers can restrict a task's migration
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to improve NUMA locality using various CPU affinity command line interfaces,
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such as taskset(1) and numactl(1), and program interfaces such as
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sched_setaffinity(2). Further, one can modify the kernel's default local
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allocation behavior using Linux NUMA memory policy.
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[see Documentation/vm/numa_memory_policy.txt.]
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System administrators can restrict the CPUs and nodes' memories that a non-
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privileged user can specify in the scheduling or NUMA commands and functions
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using control groups and CPUsets. [see Documentation/cgroup-v1/cpusets.txt]
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On architectures that do not hide memoryless nodes, Linux will include only
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zones [nodes] with memory in the zonelists. This means that for a memoryless
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node the "local memory node"--the node of the first zone in CPU's node's
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zonelist--will not be the node itself. Rather, it will be the node that the
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kernel selected as the nearest node with memory when it built the zonelists.
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So, default, local allocations will succeed with the kernel supplying the
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closest available memory. This is a consequence of the same mechanism that
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allows such allocations to fallback to other nearby nodes when a node that
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does contain memory overflows.
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Some kernel allocations do not want or cannot tolerate this allocation fallback
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behavior. Rather they want to be sure they get memory from the specified node
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or get notified that the node has no free memory. This is usually the case when
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a subsystem allocates per CPU memory resources, for example.
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A typical model for making such an allocation is to obtain the node id of the
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node to which the "current CPU" is attached using one of the kernel's
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numa_node_id() or CPU_to_node() functions and then request memory from only
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the node id returned. When such an allocation fails, the requesting subsystem
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may revert to its own fallback path. The slab kernel memory allocator is an
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example of this. Or, the subsystem may choose to disable or not to enable
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itself on allocation failure. The kernel profiling subsystem is an example of
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this.
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If the architecture supports--does not hide--memoryless nodes, then CPUs
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attached to memoryless nodes would always incur the fallback path overhead
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or some subsystems would fail to initialize if they attempted to allocated
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memory exclusively from a node without memory. To support such
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architectures transparently, kernel subsystems can use the numa_mem_id()
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or cpu_to_mem() function to locate the "local memory node" for the calling or
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specified CPU. Again, this is the same node from which default, local page
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allocations will be attempted.
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