Re: [PATCH] mm: swap: async free swap slot cache entries

From: Nhat Pham
Date: Fri Dec 22 2023 - 20:44:39 EST

On Thu, Dec 21, 2023 at 10:25 PM Chris Li <chrisl@xxxxxxxxxx> wrote:
>
> We discovered that 1% swap page fault is 100us+ while 50% of
> the swap fault is under 20us.
>
> Further investigation show that a large portion of the time
> spent in the free_swap_slots() function for the long tail case.
>
> The percpu cache of swap slots is freed in a batch of 64 entries
> inside free_swap_slots(). These cache entries are accumulated
> from previous page faults, which may not be related to the current
> process.
>
> Doing the batch free in the page fault handler causes longer
> tail latencies and penalizes the current process.
>
> Move free_swap_slots() outside of the swapin page fault handler into an
> async work queue to avoid such long tail latencies.
>
> Testing:
>
> Chun-Tse did some benchmark in chromebook, showing that
> zram_wait_metrics improve about 15% with 80% and 95% confidence.
>
> I recently ran some experiments on about 1000 Google production
> machines. It shows swapin latency drops in the long tail
> 100us - 500us bucket dramatically.
>
> platform (100-500us) (0-100us)
> A 1.12% -> 0.36% 98.47% -> 99.22%
> B 0.65% -> 0.15% 98.96% -> 99.46%
> C 0.61% -> 0.23% 98.96% -> 99.38%

Nice! Are these values for zram as well, or ordinary (SSD?) swap? I
imagine it will matter less for swap, right?

>
> Signed-off-by: Chris Li <chrisl@xxxxxxxxxx>
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: Wei Xu <weixugc@xxxxxxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> Cc: Greg Thelen <gthelen@xxxxxxxxxx>
> Cc: Chun-Tse Shao <ctshao@xxxxxxxxxx>
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Cc: Brain Geffon <bgeffon@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Huang Ying <ying.huang@xxxxxxxxx>
> Cc: Nhat Pham <nphamcs@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Kairui Song <kasong@xxxxxxxxxxx>
> Cc: Zhongkun He <hezhongkun.hzk@xxxxxxxxxxxxx>
> Cc: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
> Cc: Barry Song <v-songbaohua@xxxxxxxx>
> ---
> include/linux/swap_slots.h | 1 +
> mm/swap_slots.c | 37 +++++++++++++++++++++++++++++--------
> 2 files changed, 30 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/swap_slots.h b/include/linux/swap_slots.h
> index 15adfb8c813a..67bc8fa30d63 100644
> --- a/include/linux/swap_slots.h
> +++ b/include/linux/swap_slots.h
> @@ -19,6 +19,7 @@ struct swap_slots_cache {
> spinlock_t free_lock; /* protects slots_ret, n_ret */
> swp_entry_t *slots_ret;
> int n_ret;
> + struct work_struct async_free;
> };
>
> void disable_swap_slots_cache_lock(void);
> diff --git a/mm/swap_slots.c b/mm/swap_slots.c
> index 0bec1f705f8e..a3b306550732 100644
> --- a/mm/swap_slots.c
> +++ b/mm/swap_slots.c
> @@ -42,8 +42,10 @@ static bool swap_slot_cache_initialized;
> static DEFINE_MUTEX(swap_slots_cache_mutex);
> /* Serialize swap slots cache enable/disable operations */
> static DEFINE_MUTEX(swap_slots_cache_enable_mutex);
> +static struct workqueue_struct *swap_free_queue;
>
> static void __drain_swap_slots_cache(unsigned int type);
> +static void swapcache_async_free_entries(struct work_struct *data);
>
> #define use_swap_slot_cache (swap_slot_cache_active && swap_slot_cache_enabled)
> #define SLOTS_CACHE 0x1
> @@ -149,6 +151,7 @@ static int alloc_swap_slot_cache(unsigned int cpu)
> spin_lock_init(&cache->free_lock);
> cache->lock_initialized = true;
> }
> + INIT_WORK(&cache->async_free, swapcache_async_free_entries);
> cache->nr = 0;
> cache->cur = 0;
> cache->n_ret = 0;
> @@ -269,6 +272,20 @@ static int refill_swap_slots_cache(struct swap_slots_cache *cache)
> return cache->nr;
> }
>
> +static void swapcache_async_free_entries(struct work_struct *data)
> +{
> + struct swap_slots_cache *cache;
> +
> + cache = container_of(data, struct swap_slots_cache, async_free);
> + spin_lock_irq(&cache->free_lock);
> + /* Swap slots cache may be deactivated before acquiring lock */
> + if (cache->slots_ret) {
> + swapcache_free_entries(cache->slots_ret, cache->n_ret);
> + cache->n_ret = 0;
> + }
> + spin_unlock_irq(&cache->free_lock);
> +}
> +
> void free_swap_slot(swp_entry_t entry)
> {
> struct swap_slots_cache *cache;
> @@ -282,17 +299,14 @@ void free_swap_slot(swp_entry_t entry)
> goto direct_free;
> }
> if (cache->n_ret >= SWAP_SLOTS_CACHE_SIZE) {
> - /*
> - * Return slots to global pool.
> - * The current swap_map value is SWAP_HAS_CACHE.
> - * Set it to 0 to indicate it is available for
> - * allocation in global pool
> - */
> - swapcache_free_entries(cache->slots_ret, cache->n_ret);
> - cache->n_ret = 0;
> + spin_unlock_irq(&cache->free_lock);
> + queue_work(swap_free_queue, &cache->async_free);
> + goto direct_free;
> }
> cache->slots_ret[cache->n_ret++] = entry;
> spin_unlock_irq(&cache->free_lock);
> + if (cache->n_ret >= SWAP_SLOTS_CACHE_SIZE)
> + queue_work(swap_free_queue, &cache->async_free);
> } else {
> direct_free:
> swapcache_free_entries(&entry, 1);
> @@ -348,3 +362,10 @@ swp_entry_t folio_alloc_swap(struct folio *folio)
> }
> return entry;
> }
> +
> +static int __init async_queue_init(void)
> +{
> + swap_free_queue = create_workqueue("async swap cache");

nit(?): isn't create_workqueue() deprecated? from:

https://www.kernel.org/doc/html/latest/core-api/workqueue.html#application-programming-interface-api

I think there's a zswap patch proposing fixing that on the zswap side.

> + return 0;
> +}
> +subsys_initcall(async_queue_init);
>
> ---
> base-commit: eacce8189e28717da6f44ee492b7404c636ae0de
> change-id: 20231216-async-free-bef392015432
>
> Best regards,
> --
> Chris Li <chrisl@xxxxxxxxxx>
>