[PATCH v4 0/2] nvme/pci: PRP list DMA pool partitioning

[Caleb Sander Mateos <csander@purestorage.com>]

NVMe commands with more than 4 KB of 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.

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

 drivers/nvme/host/pci.c | 170 +++++++++++++++++++++++-----------------
 1 file changed, 97 insertions(+), 73 deletions(-)

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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