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

[Qian Cai <cai@lca.pw>]

> On Sep 11, 2019, at 4:54 PM, Waiman Long <longman@redhat.com> wrote:
> 
> On 9/11/19 8:42 PM, Qian Cai wrote:
>> 
>>> 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.
> 
> Right, I understand that. I will move to use a sysctl parameters for the
> timeout and then set its default value to either 10ms or 20ms if some
> debug options are detected. Usually the slower than should not be more
> than 2X.

That 2X is another magic number which has no testing data back for it. We need a way to disable timeout
completely in Kconfig, so it can ship in the part of a debug kernel package.