Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge PMD

[Qian Cai <cai@lca.pw>]

> On Sep 11, 2019, at 11:05 AM, Waiman Long <longman@redhat.com> wrote:
> 
> When allocating a large amount of static hugepages (~500-1500GB) on a
> system with large number of CPUs (4, 8 or even 16 sockets), performance
> degradation (random multi-second delays) was observed when thousands
> of processes are trying to fault in the data into the huge pages. The
> likelihood of the delay increases with the number of sockets and hence
> the CPUs a system has.  This only happens in the initial setup phase
> and will be gone after all the necessary data are faulted in.
> 
> These random delays, however, are deemed unacceptable. The cause of
> that delay is the long wait time in acquiring the mmap_sem when trying
> to share the huge PMDs.
> 
> To remove the unacceptable delays, we have to limit the amount of wait
> time on the mmap_sem. So the new down_write_timedlock() function is
> used to acquire the write lock on the mmap_sem with a timeout value of
> 10ms which should not cause a perceivable delay. If timeout happens,
> the task will abandon its effort to share the PMD and allocate its own
> copy instead.
> 
> When too many timeouts happens (threshold currently set at 256), the
> system may be too large for PMD sharing to be useful without undue delay.
> So the sharing will be disabled in this case.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
> include/linux/fs.h |  7 +++++++
> mm/hugetlb.c       | 24 +++++++++++++++++++++---
> 2 files changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 997a530ff4e9..e9d3ad465a6b 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -40,6 +40,7 @@
> #include <linux/fs_types.h>
> #include <linux/build_bug.h>
> #include <linux/stddef.h>
> +#include <linux/ktime.h>
> 
> #include <asm/byteorder.h>
> #include <uapi/linux/fs.h>
> @@ -519,6 +520,12 @@ static inline void i_mmap_lock_write(struct address_space *mapping)
> 	down_write(&mapping->i_mmap_rwsem);
> }
> 
> +static inline bool i_mmap_timedlock_write(struct address_space *mapping,
> +					 ktime_t timeout)
> +{
> +	return down_write_timedlock(&mapping->i_mmap_rwsem, timeout);
> +}
> +
> static inline void i_mmap_unlock_write(struct address_space *mapping)
> {
> 	up_write(&mapping->i_mmap_rwsem);
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 6d7296dd11b8..445af661ae29 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4750,6 +4750,8 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
> 	}
> }
> 
> +#define PMD_SHARE_DISABLE_THRESHOLD	(1 << 8)
> +
> /*
>  * Search for a shareable pmd page for hugetlb. In any case calls pmd_alloc()
>  * and returns the corresponding pte. While this is not necessary for the
> @@ -4770,11 +4772,24 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
> 	pte_t *spte = NULL;
> 	pte_t *pte;
> 	spinlock_t *ptl;
> +	static atomic_t timeout_cnt;
> 
> -	if (!vma_shareable(vma, addr))
> -		return (pte_t *)pmd_alloc(mm, pud, addr);
> +	/*
> +	 * Don't share if it is not sharable or locking attempt timed out
> +	 * after 10ms. After 256 timeouts, PMD sharing will be permanently
> +	 * disabled as it is just too slow.

It looks like this kind of policy interacts with kernel debug options like KASAN (which is going to slow the system down
anyway) could introduce tricky issues due to different timings on a debug kernel.

> +	 */
> +	if (!vma_shareable(vma, addr) ||
> +	   (atomic_read(&timeout_cnt) >= PMD_SHARE_DISABLE_THRESHOLD))
> +		goto out_no_share;
> +
> +	if (!i_mmap_timedlock_write(mapping, ms_to_ktime(10))) {
> +		if (atomic_inc_return(&timeout_cnt) ==
> +		    PMD_SHARE_DISABLE_THRESHOLD)
> +			pr_info("Hugetlbfs PMD sharing disabled because of timeouts!\n");
> +		goto out_no_share;
> +	}
> 
> -	i_mmap_lock_write(mapping);
> 	vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
> 		if (svma == vma)
> 			continue;
> @@ -4806,6 +4821,9 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
> 	pte = (pte_t *)pmd_alloc(mm, pud, addr);
> 	i_mmap_unlock_write(mapping);
> 	return pte;
> +
> +out_no_share:
> +	return (pte_t *)pmd_alloc(mm, pud, addr);
> }
> 
> /*
> -- 
> 2.18.1
> 
>