On 11/7/19 2:06 PM, Waiman Long wrote:
> A customer with large SMP systems (up to 16 sockets) with application
> that uses large amount of static hugepages (~500-1500GB) are experiencing
> random multisecond delays. These delays was caused by the long time it
> took to scan the VMA interval tree with mmap_sem held.
>
> The sharing of huge PMD does not require changes to the i_mmap at all.
> As a result, we can just take the read lock and let other threads
> searching for the right VMA to share in parallel. Once the right
> VMA is found, either the PMD lock (2M huge page for x86-64) or the
> mm->page_table_lock will be acquired to perform the actual PMD sharing.
>
> Lock contention, if present, will happen in the spinlock. That is much
> better than contention in the rwsem where the time needed to scan the
> the interval tree is indeterminate.
>
> With this patch applied, the customer is seeing significant improvements
> over the unpatched kernel.
Oh, I forgot to add
Suggested-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
> mm/hugetlb.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index b45a95363a84..087e7ff00137 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4842,7 +4842,11 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
> if (!vma_shareable(vma, addr))
> return (pte_t *)pmd_alloc(mm, pud, addr);
>
> - i_mmap_lock_write(mapping);
> + /*
> + * PMD sharing does not require changes to i_mmap. So a read lock
> + * is enuogh.
> + */
> + i_mmap_lock_read(mapping);
> vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
> if (svma == vma)
> continue;
> @@ -4872,7 +4876,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
> spin_unlock(ptl);
> out:
> pte = (pte_t *)pmd_alloc(mm, pud, addr);
> - i_mmap_unlock_write(mapping);
> + i_mmap_unlock_read(mapping);
> return pte;
> }
>
Cheers,
Longman
