Re: [PATCH 4/4] swap: change swap_list_head to plist, add swap_avail_head

[Steven Rostedt]

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.

I know Peter Zijlstra was doing some work to convert the rtmutex code to use
rb-trees instead of plists. Peter is that moving forward?

If there are other users of plists we should remove the dependency from the 
DEBUG_RT_MUTEX and DEBUG_PI_LIST.

> 
> Also, I might have over-commented in this patch; if so I can remove/reduce
> some of it. :)

"over-commented"?? There's no such word ;-)

-- Steve

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