Re: [PATCH v5 3/3] nvme/pci: make PRP list DMA pools per-NUMA-node

[Christoph Hellwig <hch@lst.de>]

On Tue, Apr 22, 2025 at 04:09:52PM -0600, Caleb Sander Mateos wrote:
> 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.

That's not actually true.  We can transfer all of the MDTS without a
single pool allocation when using SGLs.

> 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.

Should we try to simply do a slab allocation first and only allocate
from the dmapool when that fails?  That should give you all the
scalability from the slab allocator without very little downsides.