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

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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@xxxxxxxxx>
> Tested-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
>  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?

>  		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
> 
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov




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