linux-kernel - Re: [PATCH v5 0/3] nvme/pci: PRP list DMA pool partitioning

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Message-ID: <09bde11c-a3f3-4c5a-91ed-74bfd2e0f61d@kernel.dk>
Date: Wed, 23 Apr 2025 07:21:56 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Caleb Sander Mateos <csander@...estorage.com>,
 Keith Busch <kbusch@...nel.org>, Christoph Hellwig <hch@....de>,
 Sagi Grimberg <sagi@...mberg.me>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Kanchan Joshi <joshi.k@...sung.com>, linux-nvme@...ts.infradead.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/3] nvme/pci: PRP list DMA pool partitioning

On 4/22/25 4:09 PM, 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. 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%.

Looks good to me:

Reviewed-by: Jens Axboe <axboe@...nel.dk>

-- 
Jens Axboe