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Date: Thu, 24 Apr 2014 08:17:05 +0800
From: Shaohua Li <shli@...nel.org>
To: Mel Gorman <mgorman@...e.de>
Cc: Dan Streetman <ddstreet@...e.org>, Hugh Dickins <hughd@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Michal Hocko <mhocko@...e.cz>,
Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
Shaohua Li <shli@...ionio.com>,
Weijie Yang <weijieut@...il.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] swap: change swap_info singly-linked list to
list_head
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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