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mm/mempolicy: add MPOL_PREFERRED_MANY for multiple preferred nodes
Patch series "Introduce multi-preference mempolicy", v7. This patch series introduces the concept of the MPOL_PREFERRED_MANY mempolicy. This mempolicy mode can be used with either the set_mempolicy(2) or mbind(2) interfaces. Like the MPOL_PREFERRED interface, it allows an application to set a preference for nodes which will fulfil memory allocation requests. Unlike the MPOL_PREFERRED mode, it takes a set of nodes. Like the MPOL_BIND interface, it works over a set of nodes. Unlike MPOL_BIND, it will not cause a SIGSEGV or invoke the OOM killer if those preferred nodes are not available. Along with these patches are patches for libnuma, numactl, numademo, and memhog. They still need some polish, but can be found here: https://gitlab.com/bwidawsk/numactl/-/tree/prefer-many It allows new usage: `numactl -P 0,3,4` The goal of the new mode is to enable some use-cases when using tiered memory usage models which I've lovingly named. 1a. The Hare - The interconnect is fast enough to meet bandwidth and latency requirements allowing preference to be given to all nodes with "fast" memory. 1b. The Indiscriminate Hare - An application knows it wants fast memory (or perhaps slow memory), but doesn't care which node it runs on. The application can prefer a set of nodes and then xpu bind to the local node (cpu, accelerator, etc). This reverses the nodes are chosen today where the kernel attempts to use local memory to the CPU whenever possible. This will attempt to use the local accelerator to the memory. 2. The Tortoise - The administrator (or the application itself) is aware it only needs slow memory, and so can prefer that. Much of this is almost achievable with the bind interface, but the bind interface suffers from an inability to fallback to another set of nodes if binding fails to all nodes in the nodemask. Like MPOL_BIND a nodemask is given. Inherently this removes ordering from the preference. > /* Set first two nodes as preferred in an 8 node system. */ > const unsigned long nodes = 0x3 > set_mempolicy(MPOL_PREFER_MANY, &nodes, 8); > /* Mimic interleave policy, but have fallback *. > const unsigned long nodes = 0xaa > set_mempolicy(MPOL_PREFER_MANY, &nodes, 8); Some internal discussion took place around the interface. There are two alternatives which we have discussed, plus one I stuck in: 1. Ordered list of nodes. Currently it's believed that the added complexity is nod needed for expected usecases. 2. A flag for bind to allow falling back to other nodes. This confuses the notion of binding and is less flexible than the current solution. 3. Create flags or new modes that helps with some ordering. This offers both a friendlier API as well as a solution for more customized usage. It's unknown if it's worth the complexity to support this. Here is sample code for how this might work: > // Prefer specific nodes for some something wacky > set_mempolicy(MPOL_PREFER_MANY, 0x17c, 1024); > > // Default > set_mempolicy(MPOL_PREFER_MANY | MPOL_F_PREFER_ORDER_SOCKET, NULL, 0); > // which is the same as > set_mempolicy(MPOL_DEFAULT, NULL, 0); > > // The Hare > set_mempolicy(MPOL_PREFER_MANY | MPOL_F_PREFER_ORDER_TYPE, NULL, 0); > > // The Tortoise > set_mempolicy(MPOL_PREFER_MANY | MPOL_F_PREFER_ORDER_TYPE_REV, NULL, 0); > > // Prefer the fast memory of the first two sockets > set_mempolicy(MPOL_PREFER_MANY | MPOL_F_PREFER_ORDER_TYPE, -1, 2); > This patch (of 5): The NUMA APIs currently allow passing in a "preferred node" as a single bit set in a nodemask. If more than one bit it set, bits after the first are ignored. This single node is generally OK for location-based NUMA where memory being allocated will eventually be operated on by a single CPU. However, in systems with multiple memory types, folks want to target a *type* of memory instead of a location. For instance, someone might want some high-bandwidth memory but do not care about the CPU next to which it is allocated. Or, they want a cheap, high capacity allocation and want to target all NUMA nodes which have persistent memory in volatile mode. In both of these cases, the application wants to target a *set* of nodes, but does not want strict MPOL_BIND behavior as that could lead to OOM killer or SIGSEGV. So add MPOL_PREFERRED_MANY policy to support the multiple preferred nodes requirement. This is not a pie-in-the-sky dream for an API. This was a response to a specific ask of more than one group at Intel. Specifically: 1. There are existing libraries that target memory types such as https://github.com/memkind/memkind. These are known to suffer from SIGSEGV's when memory is low on targeted memory "kinds" that span more than one node. The MCDRAM on a Xeon Phi in "Cluster on Die" mode is an example of this. 2. Volatile-use persistent memory users want to have a memory policy which is targeted at either "cheap and slow" (PMEM) or "expensive and fast" (DRAM). However, they do not want to experience allocation failures when the targeted type is unavailable. 3. Allocate-then-run. Generally, we let the process scheduler decide on which physical CPU to run a task. That location provides a default allocation policy, and memory availability is not generally considered when placing tasks. For situations where memory is valuable and constrained, some users want to allocate memory first, *then* allocate close compute resources to the allocation. This is the reverse of the normal (CPU) model. Accelerators such as GPUs that operate on core-mm-managed memory are interested in this model. A check is added in sanitize_mpol_flags() to not permit 'prefer_many' policy to be used for now, and will be removed in later patch after all implementations for 'prefer_many' are ready, as suggested by Michal Hocko. [[email protected]: suggest to refine policy_node/policy_nodemask handling] Link: https://lkml.kernel.org/r/[email protected] Link: https://lore.kernel.org/r/[email protected] Link: https://lkml.kernel.org/r/[email protected] Co-developed-by: Ben Widawsky <[email protected]> Signed-off-by: Ben Widawsky <[email protected]> Signed-off-by: Dave Hansen <[email protected]> Signed-off-by: Feng Tang <[email protected]> Cc: Michal Hocko <[email protected]> Acked-by: Michal Hocko <[email protected]> Cc: Andrea Arcangeli <[email protected]> Cc: Mel Gorman <[email protected]> Cc: Mike Kravetz <[email protected]> Cc: Randy Dunlap <[email protected]> Cc: Vlastimil Babka <[email protected]> Cc: Andi Kleen <[email protected]> Cc: Dan Williams <[email protected]> Cc: Huang Ying <[email protected]>b Cc: Michal Hocko <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
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