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

[Qian Cai <cai@lca.pw>]

> On Sep 11, 2019, at 12:34 PM, Waiman Long <longman@redhat.com> wrote:
> 
> On 9/11/19 5:01 PM, Qian Cai wrote:
>> 
>>> 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.
> 
> With respect to lockdep, down_write_timedlock() works like a trylock. So
> a lot of checking will be skipped. Also the lockdep code won't be run
> until the lock is acquired. So its execution time has no effect on the
> timeout.

No only lockdep, but also things like KASAN, debug_pagealloc, page_poison, kmemleak, debug
objects etc that  all going to slow down things in huge_pmd_share(), and make it tricky to get a
right timeout value for those debug kernels without changing the previous behavior.