Re: [PATCH v2 1/2] THP: avoid lock when check whether THP is in deferred list

Ryan Roberts
2023-04-26T08:17:36Z

On 26/04/2023 03:08, Yin Fengwei wrote:
>
>
> On 4/25/23 20:38, Kirill A. Shutemov wrote:
>> On Tue, Apr 25, 2023 at 04:46:26PM +0800, Yin Fengwei wrote:
>>> free_transhuge_page() acquires split queue lock then check
>>> whether the THP was added to deferred list or not.
>>>
>>> It's safe to check whether the THP is in deferred list or not.
>>> When code hit free_transhuge_page(), there is no one tries
>>> to update the folio's _deferred_list.
>>>
>>> If folio is not in deferred_list, it's safe to check without
>>> acquiring lock.
>>>
>>> If folio is in deferred_list, the other node in deferred_list
>>> adding/deleteing doesn't impact the return value of
>>> list_epmty(@folio->_deferred_list).
>>
>> Typo.
>>
>>>
>>> Running page_fault1 of will-it-scale + order 2 folio for anonymous
>>> mapping with 96 processes on an Ice Lake 48C/96T test box, we could
>>> see the 61% split_queue_lock contention:
>>> - 71.28% 0.35% page_fault1_pro [kernel.kallsyms] [k]
>>> release_pages
>>> - 70.93% release_pages
>>> - 61.42% free_transhuge_page
>>> + 60.77% _raw_spin_lock_irqsave
>>>
>>> With this patch applied, the split_queue_lock contention is less
>>> than 1%.
>>>
>>> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
>>> Tested-by: Ryan Roberts <ryan.roberts@arm.com>
>>> ---
>>> mm/huge_memory.c | 19 ++++++++++++++++---
>>> 1 file changed, 16 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 032fb0ef9cd1..c620f1f12247 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -2799,12 +2799,25 @@ void free_transhuge_page(struct page *page)
>>> struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>>> unsigned long flags;
>>>
>>> - spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>>> - if (!list_empty(&folio->_deferred_list)) {
>>> + /*
>>> + * At this point, there is no one trying to queue the folio
>>> + * to deferred_list. folio->_deferred_list is not possible
>>> + * being updated.
>>> + *
>>> + * If folio is already added to deferred_list, add/delete to/from
>>> + * deferred_list will not impact list_empty(&folio->_deferred_list).
>>> + * It's safe to check list_empty(&folio->_deferred_list) without
>>> + * acquiring the lock.
>>> + *
>>> + * If folio is not in deferred_list, it's safe to check without
>>> + * acquiring the lock.
>>> + */
>>> + if (data_race(!list_empty(&folio->_deferred_list))) {
>>> + spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>>
>> Recheck under lock?
> Huang Ying pointed out the race with deferred_split_scan(). And Yes. Need
> recheck under lock. Will update in next version.

Oops sorry - I see this was already pointed out. Disregard my previous mail.

Thanks,
Ryan

>
>
> Regards
> Yin, Fengwei
>
>>
>>> ds_queue->split_queue_len--;
>>> list_del(&folio->_deferred_list);
>>> + spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>>> }
>>> - spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>>> free_compound_page(page);
>>> }
>>>
>>> --
>>> 2.30.2
>>>
>>>
>>