On Fri, May 02, 2014 at 03:02:30PM -0400, Dan Streetman wrote: > Originally get_swap_page() started iterating through the singly-linked > list of swap_info_structs using swap_list.next or highest_priority_index, > which both were intended to point to the highest priority active swap > target that was not full. The first patch in this series changed the > singly-linked list to a doubly-linked list, and removed the logic to start > at the highest priority non-full entry; it starts scanning at the highest > priority entry each time, even if the entry is full. > > Replace the manually ordered swap_list_head with a plist, renamed to > swap_active_head for clarity. Add a new plist, swap_avail_head. > The original swap_active_head plist contains all active swap_info_structs, > as before, while the new swap_avail_head plist contains only > swap_info_structs that are active and available, i.e. not full. > Add a new spinlock, swap_avail_lock, to protect the swap_avail_head list. > > Mel Gorman suggested using plists since they internally handle ordering > the list entries based on priority, which is exactly what swap was doing > manually. All the ordering code is now removed, and swap_info_struct > entries and simply added to their corresponding plist and automatically > ordered correctly. > > Using a new plist for available swap_info_structs simplifies and > optimizes get_swap_page(), which no longer has to iterate over full > swap_info_structs. Using a new spinlock for swap_avail_head plist > allows each swap_info_struct to add or remove themselves from the > plist when they become full or not-full; previously they could not > do so because the swap_info_struct->lock is held when they change > from full<->not-full, and the swap_lock protecting the main > swap_active_head must be ordered before any swap_info_struct->lock. > > Signed-off-by: Dan Streetman <ddstreet@xxxxxxxx> > Cc: Mel Gorman <mgorman@xxxxxxx> > Cc: Shaohua Li <shli@xxxxxxxxxxxx> > > --- > > Mel, I tried moving the ordering and rotating code into common list functions > and I also tried plists, and you were right, using plists is much simpler and > more maintainable. The only required update to plist is the plist_rotate() > function, which is even simpler to use in get_swap_page() than the > list_rotate_left() function. > > After looking more closely at plists, I don't see how they would reduce > performance, so I don't think there is any concern there, although Shaohua if > you have time it might be nice to check this updated patch set's performance. > I will note that if CONFIG_DEBUG_PI_LIST is set, there's quite a lot of list > checking going on for each list modification including rotate; that config is > set if "RT Mutex debugging, deadlock detection" is set, so I assume in that > case overall system performance is expected to be less than optimal. > > Also, I might have over-commented in this patch; if so I can remove/reduce > some of it. :) > > Changelog since v1 https://lkml.org/lkml/2014/4/12/73 > -use plists instead of regular lists > -update/add comments > > include/linux/swap.h | 3 +- > include/linux/swapfile.h | 2 +- > mm/frontswap.c | 6 +- > mm/swapfile.c | 142 +++++++++++++++++++++++++++++------------------ > 4 files changed, 94 insertions(+), 59 deletions(-) > > diff --git a/include/linux/swap.h b/include/linux/swap.h > index 8bb85d6..9155bcd 100644 > --- a/include/linux/swap.h > +++ b/include/linux/swap.h > @@ -214,7 +214,8 @@ struct percpu_cluster { > struct swap_info_struct { > unsigned long flags; /* SWP_USED etc: see above */ > signed short prio; /* swap priority of this type */ > - struct list_head list; /* entry in swap list */ > + struct plist_node list; /* entry in swap_active_head */ > + struct plist_node avail_list; /* entry in swap_avail_head */ > signed char type; /* strange name for an index */ > unsigned int max; /* extent of the swap_map */ > unsigned char *swap_map; /* vmalloc'ed array of usage counts */ > diff --git a/include/linux/swapfile.h b/include/linux/swapfile.h > index 2eab382..388293a 100644 > --- a/include/linux/swapfile.h > +++ b/include/linux/swapfile.h > @@ -6,7 +6,7 @@ > * want to expose them to the dozens of source files that include swap.h > */ > extern spinlock_t swap_lock; > -extern struct list_head swap_list_head; > +extern struct plist_head swap_active_head; > extern struct swap_info_struct *swap_info[]; > extern int try_to_unuse(unsigned int, bool, unsigned long); > > diff --git a/mm/frontswap.c b/mm/frontswap.c > index fae1160..c30eec5 100644 > --- a/mm/frontswap.c > +++ b/mm/frontswap.c > @@ -331,7 +331,7 @@ static unsigned long __frontswap_curr_pages(void) > struct swap_info_struct *si = NULL; > > assert_spin_locked(&swap_lock); > - list_for_each_entry(si, &swap_list_head, list) > + plist_for_each_entry(si, &swap_active_head, list) > totalpages += atomic_read(&si->frontswap_pages); > return totalpages; > } > @@ -346,7 +346,7 @@ static int __frontswap_unuse_pages(unsigned long total, unsigned long *unused, > unsigned long pages = 0, pages_to_unuse = 0; > > assert_spin_locked(&swap_lock); > - list_for_each_entry(si, &swap_list_head, list) { > + plist_for_each_entry(si, &swap_active_head, list) { > si_frontswap_pages = atomic_read(&si->frontswap_pages); > if (total_pages_to_unuse < si_frontswap_pages) { > pages = pages_to_unuse = total_pages_to_unuse; > @@ -408,7 +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_active_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 6c95a8c..ec230e3 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -61,7 +61,22 @@ static const char Unused_offset[] = "Unused swap offset entry "; > * all active swap_info_structs > * protected with swap_lock, and ordered by priority. > */ > -LIST_HEAD(swap_list_head); > +PLIST_HEAD(swap_active_head); > + > +/* > + * all available (active, not full) swap_info_structs > + * protected with swap_avail_lock, ordered by priority. > + * This is used by get_swap_page() instead of swap_active_head > + * because swap_active_head includes all swap_info_structs, > + * but get_swap_page() doesn't need to look at full ones. > + * This uses its own lock instead of swap_lock because when a > + * swap_info_struct changes between not-full/full, it needs to > + * add/remove itself to/from this list, but the swap_info_struct->lock > + * is held and the locking order requires swap_lock to be taken > + * before any swap_info_struct->lock. > + */ > +static PLIST_HEAD(swap_avail_head); > +static DEFINE_SPINLOCK(swap_avail_lock); > > struct swap_info_struct *swap_info[MAX_SWAPFILES]; > > @@ -594,6 +609,9 @@ checks: > if (si->inuse_pages == si->pages) { > si->lowest_bit = si->max; > si->highest_bit = 0; > + spin_lock(&swap_avail_lock); > + plist_del(&si->avail_list, &swap_avail_head); > + spin_unlock(&swap_avail_lock); > } > si->swap_map[offset] = usage; > inc_cluster_info_page(si, si->cluster_info, offset); > @@ -645,57 +663,60 @@ swp_entry_t get_swap_page(void) > { > struct swap_info_struct *si, *next; > pgoff_t offset; > - struct list_head *tmp; > > - spin_lock(&swap_lock); > if (atomic_long_read(&nr_swap_pages) <= 0) > goto noswap; > atomic_long_dec(&nr_swap_pages); > > - list_for_each(tmp, &swap_list_head) { > - si = list_entry(tmp, typeof(*si), list); > + spin_lock(&swap_avail_lock); > +start_over: > + plist_for_each_entry_safe(si, next, &swap_avail_head, avail_list) { > + /* rotate si to tail of same-priority siblings */ > + plist_rotate(&si->avail_list, &swap_avail_head); > + spin_unlock(&swap_avail_lock); > spin_lock(&si->lock); > if (!si->highest_bit || !(si->flags & SWP_WRITEOK)) { > + spin_lock(&swap_avail_lock); > + if (plist_node_empty(&si->avail_list)) { > + spin_unlock(&si->lock); > + goto nextsi; > + } It's a corner case but rather than dropping the swap_avail_lock early and retaking it to remove an entry from avail_list, you could just drop it after this check but before the scan_swap_map. It's not a big deal so whether you change this or not Acked-by: Mel Gorman <mgorman@xxxxxxx> -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>