vfio/type1: Add conditional rescheduling

IOMMU operations can be expensive and it's not very difficult for a
user to give us a lot of work to do for a map or unmap operation.
Killing a large VM will vfio assigned devices can result in soft
lockups and IOMMU tracing shows that we can easily spend 80% of our
time with need-resched set.  A sprinkling of conf_resched() calls
after map and unmap calls has a very tiny affect on performance
while resulting in traces with <1% of calls overflowing into needs-
resched.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 35c90089478e..57d8c37a002b 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -351,8 +351,10 @@ static void vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma)
         domain = d = list_first_entry(&iommu->domain_list,
                                       struct vfio_domain, next);
 
-        list_for_each_entry_continue(d, &iommu->domain_list, next)
+        list_for_each_entry_continue(d, &iommu->domain_list, next) {
                 iommu_unmap(d->domain, dma->iova, dma->size);
+                cond_resched();
+        }
 
         while (iova < end) {
                 size_t unmapped, len;
@@ -384,6 +386,8 @@ static void vfio_unmap_unpin(struct vfio_iommu *iommu, struct vfio_dma *dma)
                                              unmapped >> PAGE_SHIFT,
                                              dma->prot, false);
                 iova += unmapped;
+
+                cond_resched();
         }
 
         vfio_lock_acct(-unlocked);
@@ -528,6 +532,8 @@ static int vfio_iommu_map(struct vfio_iommu *iommu, dma_addr_t iova,
                             map_try_harder(d, iova, pfn, npage, prot))
                                 goto unwind;
                 }
+
+                cond_resched();
         }
 
         return 0;