bpf: lru: adjust free target to avoid global table starvation #9131
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BPF_MAP_TYPE_LRU_HASH can recycle most recent elements well before the map is full, due to percpu reservations and force shrink before neighbor stealing. Once a CPU is unable to borrow from the global map, it will once steal one elem from a neighbor and after that each time flush this one element to the global list and immediately recycle it. Batch value LOCAL_FREE_TARGET (128) will exhaust a 10K element map with 79 CPUs. CPU 79 will observe this behavior even while its neighbors hold 78 * 127 + 1 * 15 == 9921 free elements (99%). CPUs need not be active concurrently. The issue can appear with affinity migration, e.g., irqbalance. Each CPU can reserve and then hold onto its 128 elements indefinitely. Avoid global list exhaustion by limiting aggregate percpu caches to half of map size, by adjusting LOCAL_FREE_TARGET based on cpu count. This change has no effect on sufficiently large tables. Similar to LOCAL_NR_SCANS and lru->nr_scans, introduce a map variable lru->free_target. The extra field fits in a hole in struct bpf_lru. The cacheline is already warm where read in the hot path. The field is only accessed with the lru lock held. The tests are updated to pass. Test comments are extensive: updating those is left for a v2 if the approach is considered ok. Signed-off-by: Willem de Bruijn <[email protected]>
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At least one diff in series https://patchwork.kernel.org/project/netdevbpf/list/?series=972555 expired. Closing PR. |
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Pull request for series with
subject: bpf: lru: adjust free target to avoid global table starvation
version: 1
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=972555