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

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

[Leon Hwang]

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 <lance.yang@linux.dev>
Tested-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 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

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

[David Hildenbrand (Arm)]

On 8/18/26 15:50, Leon Hwang wrote:
> 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 <lance.yang@linux.dev>
> Tested-by: Lance Yang <lance.yang@linux.dev>
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> ---
>  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;

As Michal just put it:

"PREEMPT_NONE is effectivelly dead and most cond_resched will/should be
removed. Is there any reason why you are not using full preemption when
requiring low latencies?"

https://lore.kernel.org/r/aoRnUxUlgf_kRlm8@tiehlicka

-- 
Cheers,

David