Re: [RFC PATCH v2 4/5] mm: Split release_pages work into 3 passes

[Alex Shi]

在 2020/8/19 下午12:27, Alexander Duyck 写道:
> From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> 
> The release_pages function has a number of paths that end up with the
> LRU lock having to be released and reacquired. Such an example would be the
> freeing of THP pages as it requires releasing the LRU lock so that it can
> be potentially reacquired by __put_compound_page.
> 
> In order to avoid that we can split the work into 3 passes, the first
> without the LRU lock to go through and sort out those pages that are not in
> the LRU so they can be freed immediately from those that can't. The second
> pass will then go through removing those pages from the LRU in batches as
> large as a pagevec can hold before freeing the LRU lock. Once the pages have
> been removed from the LRU we can then proceed to free the remaining pages
> without needing to worry about if they are in the LRU any further.
> 
> The general idea is to avoid bouncing the LRU lock between pages and to
> hopefully aggregate the lock for up to the full page vector worth of pages.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> ---
>  mm/swap.c |  109 +++++++++++++++++++++++++++++++++++++------------------------
>  1 file changed, 67 insertions(+), 42 deletions(-)
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index fe53449fa1b8..b405f81b2c60 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -795,6 +795,54 @@ void lru_add_drain_all(void)
>  }
>  #endif
>  
> +static void __release_page(struct page *page, struct list_head *pages_to_free)
> +{
> +	if (PageCompound(page)) {
> +		__put_compound_page(page);
> +	} else {
> +		/* Clear Active bit in case of parallel mark_page_accessed */
> +		__ClearPageActive(page);
> +		__ClearPageWaiters(page);
> +
> +		list_add(&page->lru, pages_to_free);
> +	}
> +}
> +
> +static void __release_lru_pages(struct pagevec *pvec,
> +				struct list_head *pages_to_free)
> +{
> +	struct lruvec *lruvec = NULL;
> +	unsigned long flags = 0;
> +	int i;
> +
> +	/*
> +	 * The pagevec at this point should contain a set of pages with
> +	 * their reference count at 0 and the LRU flag set. We will now
> +	 * need to pull the pages from their LRU lists.
> +	 *
> +	 * We walk the list backwards here since that way we are starting at
> +	 * the pages that should be warmest in the cache.
> +	 */
> +	for (i = pagevec_count(pvec); i--;) {
> +		struct page *page = pvec->pages[i];
> +
> +		lruvec = relock_page_lruvec_irqsave(page, lruvec, &flags);

the lock bounce is better with the patch, would you like to do further
like using add_lruvecs to reduce bounce more?

Thanks
Alex

> +		VM_BUG_ON_PAGE(!PageLRU(page), page);
> +		__ClearPageLRU(page);
> +		del_page_from_lru_list(page, lruvec, page_off_lru(page));
> +	}
> +
> +	unlock_page_lruvec_irqrestore(lruvec, flags);
> +
> +	/*
> +	 * A batch of pages are no longer on the LRU list. Go through and
> +	 * start the final process of returning the deferred pages to their
> +	 * appropriate freelists.
> +	 */
> +	for (i = pagevec_count(pvec); i--;)
> +		__release_page(pvec->pages[i], pages_to_free);
> +}
> +
>  /**
>   * release_pages - batched put_page()
>   * @pages: array of pages to release
> @@ -806,32 +854,24 @@ void lru_add_drain_all(void)
>  void release_pages(struct page **pages, int nr)
>  {
>  	int i;
> +	struct pagevec pvec;
>  	LIST_HEAD(pages_to_free);
> -	struct lruvec *lruvec = NULL;
> -	unsigned long flags;
> -	unsigned int lock_batch;
>  
> +	pagevec_init(&pvec);
> +
> +	/*
> +	 * We need to first walk through the list cleaning up the low hanging
> +	 * fruit and clearing those pages that either cannot be freed or that
> +	 * are non-LRU. We will store the LRU pages in a pagevec so that we
> +	 * can get to them in the next pass.
> +	 */
>  	for (i = 0; i < nr; i++) {
>  		struct page *page = pages[i];
>  
> -		/*
> -		 * Make sure the IRQ-safe lock-holding time does not get
> -		 * excessive with a continuous string of pages from the
> -		 * same lruvec. The lock is held only if lruvec != NULL.
> -		 */
> -		if (lruvec && ++lock_batch == SWAP_CLUSTER_MAX) {
> -			unlock_page_lruvec_irqrestore(lruvec, flags);
> -			lruvec = NULL;
> -		}
> -
>  		if (is_huge_zero_page(page))
>  			continue;
>  
>  		if (is_zone_device_page(page)) {
> -			if (lruvec) {
> -				unlock_page_lruvec_irqrestore(lruvec, flags);
> -				lruvec = NULL;
> -			}
>  			/*
>  			 * ZONE_DEVICE pages that return 'false' from
>  			 * put_devmap_managed_page() do not require special
> @@ -848,36 +888,21 @@ void release_pages(struct page **pages, int nr)
>  		if (!put_page_testzero(page))
>  			continue;
>  
> -		if (PageCompound(page)) {
> -			if (lruvec) {
> -				unlock_page_lruvec_irqrestore(lruvec, flags);
> -				lruvec = NULL;
> -			}
> -			__put_compound_page(page);
> +		if (!PageLRU(page)) {
> +			__release_page(page, &pages_to_free);
>  			continue;
>  		}
>  
> -		if (PageLRU(page)) {
> -			struct lruvec *prev_lruvec = lruvec;
> -
> -			lruvec = relock_page_lruvec_irqsave(page, lruvec,
> -									&flags);
> -			if (prev_lruvec != lruvec)
> -				lock_batch = 0;
> -
> -			VM_BUG_ON_PAGE(!PageLRU(page), page);
> -			__ClearPageLRU(page);
> -			del_page_from_lru_list(page, lruvec, page_off_lru(page));
> +		/* record page so we can get it in the next pass */
> +		if (!pagevec_add(&pvec, page)) {
> +			__release_lru_pages(&pvec, &pages_to_free);
> +			pagevec_reinit(&pvec);
>  		}
> -
> -		/* Clear Active bit in case of parallel mark_page_accessed */
> -		__ClearPageActive(page);
> -		__ClearPageWaiters(page);
> -
> -		list_add(&page->lru, &pages_to_free);
>  	}
> -	if (lruvec)
> -		unlock_page_lruvec_irqrestore(lruvec, flags);
> +
> +	/* flush any remaining LRU pages that need to be processed */
> +	if (pagevec_count(&pvec))
> +		__release_lru_pages(&pvec, &pages_to_free);
>  
>  	mem_cgroup_uncharge_list(&pages_to_free);
>  	free_unref_page_list(&pages_to_free);
>