Re: [PATCH 1/2] swap: change swap_info singly-linked list to list_head

[Shaohua Li]

On Wed, Apr 23, 2014 at 11:34:00AM +0100, Mel Gorman wrote:
> > @@ -366,7 +361,7 @@ static int __frontswap_unuse_pages(unsigned long total, unsigned long *unused,
> >  		}
> >  		vm_unacct_memory(pages);
> >  		*unused = pages_to_unuse;
> > -		*swapid = type;
> > +		*swapid = si->type;
> >  		ret = 0;
> >  		break;
> >  	}
> > @@ -413,7 +408,7 @@ void frontswap_shrink(unsigned long target_pages)
> >  	/*
> >  	 * we don't want to hold swap_lock while doing a very
> >  	 * lengthy try_to_unuse, but swap_list may change
> > -	 * so restart scan from swap_list.head each time
> > +	 * so restart scan from swap_list_head each time
> >  	 */
> >  	spin_lock(&swap_lock);
> >  	ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 4a7f7e6..b958645 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -51,14 +51,14 @@ atomic_long_t nr_swap_pages;
> >  /* protected with swap_lock. reading in vm_swap_full() doesn't need lock */
> >  long total_swap_pages;
> >  static int least_priority;
> > -static atomic_t highest_priority_index = ATOMIC_INIT(-1);
> >  
> >  static const char Bad_file[] = "Bad swap file entry ";
> >  static const char Unused_file[] = "Unused swap file entry ";
> >  static const char Bad_offset[] = "Bad swap offset entry ";
> >  static const char Unused_offset[] = "Unused swap offset entry ";
> >  
> > -struct swap_list_t swap_list = {-1, -1};
> > +/* all active swap_info */
> > +LIST_HEAD(swap_list_head);
> >  
> >  struct swap_info_struct *swap_info[MAX_SWAPFILES];
> >  
> > @@ -640,66 +640,50 @@ no_page:
> >  
> >  swp_entry_t get_swap_page(void)
> >  {
> > -	struct swap_info_struct *si;
> > +	struct swap_info_struct *si, *next;
> >  	pgoff_t offset;
> > -	int type, next;
> > -	int wrapped = 0;
> > -	int hp_index;
> > +	struct list_head *tmp;
> >  
> >  	spin_lock(&swap_lock);
> >  	if (atomic_long_read(&nr_swap_pages) <= 0)
> >  		goto noswap;
> >  	atomic_long_dec(&nr_swap_pages);
> >  
> > -	for (type = swap_list.next; type >= 0 && wrapped < 2; type = next) {
> > -		hp_index = atomic_xchg(&highest_priority_index, -1);
> > -		/*
> > -		 * highest_priority_index records current highest priority swap
> > -		 * type which just frees swap entries. If its priority is
> > -		 * higher than that of swap_list.next swap type, we use it.  It
> > -		 * isn't protected by swap_lock, so it can be an invalid value
> > -		 * if the corresponding swap type is swapoff. We double check
> > -		 * the flags here. It's even possible the swap type is swapoff
> > -		 * and swapon again and its priority is changed. In such rare
> > -		 * case, low prority swap type might be used, but eventually
> > -		 * high priority swap will be used after several rounds of
> > -		 * swap.
> > -		 */
> > -		if (hp_index != -1 && hp_index != type &&
> > -		    swap_info[type]->prio < swap_info[hp_index]->prio &&
> > -		    (swap_info[hp_index]->flags & SWP_WRITEOK)) {
> > -			type = hp_index;
> > -			swap_list.next = type;
> > -		}
> > -
> > -		si = swap_info[type];
> > -		next = si->next;
> > -		if (next < 0 ||
> > -		    (!wrapped && si->prio != swap_info[next]->prio)) {
> > -			next = swap_list.head;
> > -			wrapped++;
> > -		}
> > -
> > +	list_for_each(tmp, &swap_list_head) {
> > +		si = list_entry(tmp, typeof(*si), list);
> >  		spin_lock(&si->lock);
> > -		if (!si->highest_bit) {
> > -			spin_unlock(&si->lock);
> > -			continue;
> > -		}
> > -		if (!(si->flags & SWP_WRITEOK)) {
> > +		if (!si->highest_bit || !(si->flags & SWP_WRITEOK)) {
> >  			spin_unlock(&si->lock);
> >  			continue;
> >  		}
> >  
> > -		swap_list.next = next;
> > +		/*
> > +		 * rotate the current swap_info that we're going to use
> > +		 * to after any other swap_info that have the same prio,
> > +		 * so that all equal-priority swap_info get used equally
> > +		 */
> > +		next = si;
> > +		list_for_each_entry_continue(next, &swap_list_head, list) {
> > +			if (si->prio != next->prio)
> > +				break;
> > +			list_rotate_left(&si->list);
> > +			next = si;
> > +		}
> >  
> 
> The list manipulations will be a lot of cache writes as the list is shuffled
> around. On slow storage I do not think this will be noticable but it may
> be noticable on faster swap devices that are SSD based. I've added Shaohua
> Li to the cc as he has been concerned with the performance of swap in the
> past. Shaohua, can you run this patchset through any of your test cases
> with the addition that multiple swap files are used to see if the cache
> writes are noticable? You'll need multiple swap files, some of which are
> at equal priority so the list shuffling logic is triggered.

get_swap_page isn't hot so far (and we hold the swap_lock, which isn't
contended), guess it's because other problems hide it, for example tlb flush
overhead.

Thanks,
Shaohua
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