From: Waiman Long <longman@redhat.com>
To: Mike Kravetz <mike.kravetz@oracle.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-mm@kvack.org, Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge PMD
Date: Thu, 12 Sep 2019 10:06:14 +0100 [thread overview]
Message-ID: <b7d7d109-03cf-d750-3a56-a95837998372@redhat.com> (raw)
In-Reply-To: <ae7edcb8-74e5-037c-17e7-01b3cf9320af@oracle.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
next prev parent reply other threads:[~2019-09-12 9:06 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-11 15:05 [PATCH 0/5] hugetlbfs: Disable PMD sharing for large systems Waiman Long
2019-09-11 15:05 ` [PATCH 1/5] locking/rwsem: Add down_write_timedlock() Waiman Long
2019-09-11 15:05 ` [PATCH 2/5] locking/rwsem: Enable timeout check when spinning on owner Waiman Long
2019-09-11 15:05 ` [PATCH 3/5] locking/osq: Allow early break from OSQ Waiman Long
2019-09-11 15:05 ` [PATCH 4/5] locking/rwsem: Enable timeout check when staying in the OSQ Waiman Long
2019-09-11 15:05 ` [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge PMD Waiman Long
2019-09-11 15:14 ` Matthew Wilcox
2019-09-11 15:44 ` Waiman Long
2019-09-11 17:03 ` Mike Kravetz
2019-09-11 17:15 ` Waiman Long
2019-09-11 17:22 ` Qian Cai
2019-09-11 17:28 ` Waiman Long
2019-09-11 16:01 ` Qian Cai
2019-09-11 16:34 ` Waiman Long
2019-09-11 19:42 ` Qian Cai
2019-09-11 20:54 ` Waiman Long
2019-09-11 21:57 ` Qian Cai
2019-09-11 19:57 ` Matthew Wilcox
2019-09-11 20:51 ` Waiman Long
2019-09-12 3:26 ` Mike Kravetz
2019-09-12 3:41 ` Matthew Wilcox
2019-09-12 4:40 ` Davidlohr Bueso
2019-09-16 13:53 ` Waiman Long
2019-09-12 9:06 ` Waiman Long [this message]
2019-09-12 16:43 ` Mike Kravetz
2019-09-13 18:23 ` Waiman Long
2019-09-13 1:50 ` [PATCH 0/5] hugetlbfs: Disable PMD sharing for large systems Dave Chinner
2019-09-25 8:35 ` Peter Zijlstra
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