Date:	Mon, 12 May 2014 09:00:14 -0400
From:	Dan Streetman <ddstreet@...e.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Weijie Yang <weijieut@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Shaohua Li <shli@...ionio.com>
Subject: Re: [PATCH 4/4] swap: change swap_list_head to plist, add swap_avail_head

On Mon, May 12, 2014 at 7:11 AM, Mel Gorman <mgorman@...e.de> wrote:
> 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@...e.org>
>> Cc: Mel Gorman <mgorman@...e.de>
>> Cc: Shaohua Li <shli@...ionio.com>
>>
>> ---
>>
>> 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

Well, the locking order requires si->lock before swap_avail_lock;
that's the primary reason for using the new lock instead of swap_lock,
so that during swap_entry_free(), while holding the si->lock, the
swap_avail_lock can be taken and the si removed from swap_avail_head.
So the swap_avail_lock has to be released before taking the si->lock,
and a side effect of that is the si might be removed from
swap_avail_head list between when we release swap_avail_lock and take
si->lock, in which case we just move on to the next si.

And as you said, it's definitely a corner case.


>
> Acked-by: Mel Gorman <mgorman@...e.de>

Let me send one more rev (in the next hour or so) - I missed a
spin_unlock(&swap_avail_lock) in the total failure case (my bad).
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