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

[Waiman Long <longman@redhat.com>]

On 9/12/19 4:26 AM, Mike Kravetz wrote:
> 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.

If we can take the rwsem in read mode, that should solve the problem
AFAICS. As I don't have a full understanding of the history of that
code, I didn't try to do that in my patch.

Cheers,
Longman