Re: [PATCH] iommu/iova: use named kmem_cache for iova magazines

[Pasha Tatashin]

> Hmm, I did misspeak slightly (it's late and I really should have left
> this for tomorrow...) - that's 12KB per CPU *per domain*, but still that
> would seem to imply well over 100 domains if you have 242MB of magazine
> allocations while the iommu_iova cache isn't even on the charts... what
> the heck is that driver doing?
>
> (I don't necessarily disagree with the spirit of the patch BTW, I just
> really want to understand the situation that prompted it, and make sure
> we don't actually have a subtle leak somewhere.)

Hi Robin,

The following tracing is without Google TPU, simply upstream kernel:

The iova_domain_init_rcaches is called 159 with the following stack:

 iova_domain_init_rcaches
 iommu_setup_dma_ops
 amd_iommu_probe_finalize
 bus_iommu_probe
 iommu_device_register
 iommu_init_pci
 amd_iommu_init_pci
 state_next
 iommu_go_to_state
 amd_iommu_init
 pci_iommu_init
 do_one_initcall

Each time 1536K is allocated: in total 159 * 1536K = 238.5M

The allocation happens like this:
for (IOVA_RANGE_CACHE_MAX_SIZE)
    for_each_possible_cpu()
        iova_magazine_alloc
        iova_magazine_alloc

IOVA_RANGE_CACHE_MAX_SIZE = 6
ncpu = 128
sizeof (struct iova_magazine) = 1K

6 * 128 * (1K + 1K) = 1536K

Pasha