[PATCH] hugetlb: add cond_resched() to __unmap_hugepage_range()

Packet receive timeouts were traced to sparse HugeTLB unmapping in
production. A task unmapping a sparse 2.5 TiB HugeTLB mapping could
remain in kernel context for over 40 ms without reaching a scheduling
point while walking empty huge PTEs. Although hard IRQs could still be
handled, the per-CPU ksoftirqd thread and other runnable tasks could not
run during that interval, delaying NET_RX softirq work queued to
ksoftirqd.

Add cond_resched() at the beginning of the hugepage loop so ksoftirqd
and other runnable tasks can run between iterations. Testing with
PREEMPT_NONE showed that the maximum interval between scheduling points
fell from over 40 ms to below 2.5 ms. Total time spent in
__unmap_hugepage_range() remained about 36 ms.

Reported-by: Lance Yang <[email protected]>
Tested-by: Lance Yang <[email protected]>
Signed-off-by: Leon Hwang <[email protected]>
---
 mm/hugetlb.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index dded1768193a..0a91aac2369f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5233,6 +5233,8 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
 	last_addr_mask = hugetlb_mask_last_page(h);
 	address = start;
 	for (; address < end; address += sz) {
+		cond_resched();
+
 		ptep = hugetlb_walk(vma, address, sz);
 		if (!ptep) {
 			address |= last_addr_mask;
-- 
2.55.0