[PATCH v6 0/3] nvme/pci: PRP list DMA pool partitioning

Caleb Sander Mateos
2025-04-26T02:08:00Z

NVMe commands with over 8 KB of discontiguous data allocate PRP list
pages from the per-nvme_device dma_pool prp_page_pool or prp_small_pool.
Each call to dma_pool_alloc() and dma_pool_free() takes the per-dma_pool
spinlock. These device-global spinlocks are a significant source of
contention when many CPUs are submitting to the same NVMe devices. On a
workload issuing 32 KB reads from 16 CPUs (8 hypertwin pairs) across 2
NUMA nodes to 23 NVMe devices, we observed 2.4% of CPU time spent in
_raw_spin_lock_irqsave called from dma_pool_alloc and dma_pool_free.

Ideally, the dma_pools would be per-hctx to minimize contention. But
that could impose considerable resource costs in a system with many NVMe
devices and CPUs.

As a compromise, allocate per-NUMA-node PRP list DMA pools. Map each
nvme_queue to the set of DMA pools corresponding to its device and its
hctx's NUMA node. This reduces the _raw_spin_lock_irqsave overhead by
about half, to 1.2%. Preventing the sharing of PRP list pages across
NUMA nodes also makes them cheaper to initialize.

Allocating the dmapool structs on the desired NUMA node further reduces
the time spent in dma_pool_alloc from 0.87% to 0.50%.

Caleb Sander Mateos (2):
nvme/pci: factor out nvme_init_hctx() helper
nvme/pci: make PRP list DMA pools per-NUMA-node

Keith Busch (1):
dmapool: add NUMA affinity support

drivers/nvme/host/pci.c | 171 +++++++++++++++++++++++-----------------
include/linux/dmapool.h | 17 +++-
mm/dmapool.c | 16 ++--
3 files changed, 121 insertions(+), 83 deletions(-)

v6:
- Clarify description of when PRP list pages are allocated (Christoph)
- Add Reviewed-by tags

v5:
- Allocate dmapool structs on desired NUMA node (Keith)
- Add Reviewed-by tags

v4:
- Drop the numa_node < nr_node_ids check (Kanchan)
- Add Reviewed-by tags

v3: simplify nvme_release_prp_pools() (Keith)

v2:
- Initialize admin nvme_queue's nvme_prp_dma_pools (Kanchan)
- Shrink nvme_dev's prp_pools array from MAX_NUMNODES to nr_node_ids (Kanchan)

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2.45.2