[PATCH] mm: add cond_resched() to free_pud_range()

[Leon Hwang]

Packet receive timeouts were seen in production. Tracing showed that an
exiting process with a sparse 2.5 TiB mapping could remain in kernel
context for over 20 ms without reaching a scheduling point while
freeing PTE page-table pages. Hard IRQs could still be handled, but the
per-CPU ksoftirqd thread and other runnable tasks could not run during
that interval, delaying NET_RX softirq work queued to ksoftirqd.

Like zap_pud_range(), add cond_resched() to free_pud_range() so
ksoftirqd and other runnable tasks can run between PUD entries. Testing
with PREEMPT_NONE showed that the maximum interval between scheduling
points fell from over 20 ms to below 2 ms.

Reported-by: Lance Yang <lance.yang@xxxxxxxxx>
Tested-by: Lance Yang <lance.yang@xxxxxxxxx>
Signed-off-by: Leon Hwang <leon.hwang@xxxxxxxxx>
---
 mm/memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 4134ac607ee0..68c15449de07 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -231,7 +231,7 @@ static inline void free_pud_range(struct mmu_gather *tlb, p4d_t *p4d,
 		if (pud_none_or_clear_bad(pud))
 			continue;
 		free_pmd_range(tlb, pud, addr, next, floor, ceiling);
-	} while (pud++, addr = next, addr != end);
+	} while (pud++, cond_resched(), addr = next, addr != end);
 
 	start &= P4D_MASK;
 	if (start < floor)
-- 
2.55.0