On Thu, 23 May 2024 14:41:12 +0100 Robin Murphy <robin.murphy@xxxxxxx> wrote: > On 23/05/2024 4:19 am, Ashish Mhetre wrote: > > The current __arm_lpae_unmap() function calls dma_sync() on individual > > PTEs after clearing them. By updating the __arm_lpae_unmap() to call > > dma_sync() once for all cleared PTEs, the overall performance can be > > improved 25% for large buffer sizes. > > Below is detailed analysis of average unmap latency(in us) with and > > without this optimization obtained by running dma_map_benchmark for > > different buffer sizes. > > > > Size Time W/O Time With % Improvement > > Optimization Optimization > > (us) (us) > > > > 4KB 3.0 3.1 -3.33 > > 1MB 250.3 187.9 24.93 > > This seems highly suspect - the smallest possible block size is 2MB so a > 1MB unmap should not be affected by this path at all. > > > 2MB 493.7 368.7 25.32 > > 4MB 974.7 723.4 25.78 > > I'm guessing this is on Tegra with the workaround to force everything to > PAGE_SIZE? In the normal case a 2MB unmap should be nominally *faster* > than 4KB, since it would also be a single PTE, but with one fewer level > of table to walk to reach it. The 25% figure is rather misleading if > it's only a mitigation of an existing erratum workaround, and the actual > impact on the majority of non-broken systems is unmeasured. > > (As an aside, I think that workaround itself is a bit broken, since at > least on Tegra234 with Cortex-A78, PAGE_SIZE could be 16KB which MMU-500 > doesn't support.) > > > Signed-off-by: Ashish Mhetre <amhetre@xxxxxxxxxx> > > --- > > drivers/iommu/io-pgtable-arm.c | 34 +++++++++++++++++++++++++--------- > > 1 file changed, 25 insertions(+), 9 deletions(-) > > > > diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c > > index 3d23b924cec1..94094b711cba 100644 > > --- a/drivers/iommu/io-pgtable-arm.c > > +++ b/drivers/iommu/io-pgtable-arm.c > > @@ -256,13 +256,15 @@ static void __arm_lpae_sync_pte(arm_lpae_iopte *ptep, int num_entries, > > sizeof(*ptep) * num_entries, DMA_TO_DEVICE); > > } > > > > -static void __arm_lpae_clear_pte(arm_lpae_iopte *ptep, struct io_pgtable_cfg *cfg) > > +static void __arm_lpae_clear_pte(arm_lpae_iopte *ptep, struct io_pgtable_cfg *cfg, int num_entries) > > { > > + int i; > > > > - *ptep = 0; > > + for (i = 0; i < num_entries; i++) > > + ptep[i] = 0; > > > > if (!cfg->coherent_walk) > > - __arm_lpae_sync_pte(ptep, 1, cfg); > > + __arm_lpae_sync_pte(ptep, num_entries, cfg); > > } > > > > static size_t __arm_lpae_unmap(struct arm_lpae_io_pgtable *data, > > @@ -633,13 +635,25 @@ static size_t __arm_lpae_unmap(struct arm_lpae_io_pgtable *data, > > if (size == ARM_LPAE_BLOCK_SIZE(lvl, data)) { > > max_entries = ARM_LPAE_PTES_PER_TABLE(data) - unmap_idx_start; > > num_entries = min_t(int, pgcount, max_entries); > > - > > - while (i < num_entries) { > > - pte = READ_ONCE(*ptep); > > + arm_lpae_iopte *pte_flush; > > + int j = 0; > > + > > + pte_flush = kvcalloc(num_entries, sizeof(*pte_flush), GFP_ATOMIC); > > kvmalloc() with GFP_ATOMIC isn't valid. However, I'm not sure if there > isn't a more fundamental problem here - Rob, Boris; was it just the map > path, or would any allocation on unmap risk the GPU reclaim deadlock > thing as well? Unmap as well, because of the 'split huge page into small pages' logic when the unmap region is not aligned on 2MB.