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

[Mike Kravetz]

On 9/11/19 8:05 AM, Waiman Long 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.
> 
<snip>
> 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.
> +	 */
> +	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);

All this got me wondering if we really need to take i_mmap_rwsem in write
mode here.  We are not changing the tree, only traversing it looking for
a suitable vma.

Unless I am missing something, the hugetlb code only ever takes the semaphore
in write mode; never read.  Could this have been the result of changing the
tree semaphore to read/write?  Instead of analyzing all the code, the easiest
and safest thing would have been to take all accesses in write mode.

I can investigate more, but wanted to ask the question in case someone already
knows.

At one time, I thought it was safe to acquire the semaphore in read mode for
huge_pmd_share, but write mode for huge_pmd_unshare.  See commit b43a99900559.
This was reverted along with another patch for other reasons.

If we change change from write to read mode, this may have significant impact
on the stalls.
-- 
Mike Kravetz