| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <0c5431d984fe518f9f9b2f85639a6fc844115deb.camel@linux.intel.com>
Date: Thu, 15 Feb 2024 10:31:41 -0800
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Chris Li <chrisl@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, Wei Xu
<weixugc@...gle.com>, Yu Zhao <yuzhao@...gle.com>, Greg Thelen
<gthelen@...gle.com>, Chun-Tse Shao <ctshao@...gle.com>, Yosry Ahmed
<yosryahmed@...gle.com>, Michal Hocko <mhocko@...e.com>, Mel Gorman
<mgorman@...hsingularity.net>, Huang Ying <ying.huang@...el.com>, Nhat Pham
<nphamcs@...il.com>, Kairui Song <kasong@...cent.com>, Barry Song
<v-songbaohua@...o.com>
Subject: Re: [PATCH v4] mm: swap: async free swap slot cache entries
On Wed, 2024-02-14 at 17:02 -0800, Chris Li wrote:
> We discovered that 1% swap page fault is 100us+ while 50% of
> the swap fault is under 20us.
>
> Further investigation shows 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.
>
> When the swap cache slot is full, schedule async free cached
> swap slots in a work queue, before the next swap fault comes in.
> If the next swap fault comes in very fast, before the async
> free gets a chance to run. It will directly free all the swap
> cache in the swap fault the same way as previously.
>
> 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%
>
> Signed-off-by: Chris Li <chrisl@...nel.org>
Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
> Changes in v4:
> - Remove the sysfs interface file, according the feedback.
> - Move the full condition test inside the spinlock.
> - Link to v3: https://lore.kernel.org/r/20240213-async-free-v3-1-b89c3cc48384@kernel.org
>
> Changes in v3:
> - Address feedback from Tim Chen, direct free path will free all swap slots.
> - Add /sys/kernel/mm/swap/swap_slot_async_fee to enable async free. Default is off.
> - Link to v2: https://lore.kernel.org/r/20240131-async-free-v2-1-525f03e07184@kernel.org
>
> Changes in v2:
> - Add description of the impact of time changing suggest by Ying.
> - Remove create_workqueue() and use schedule_work()
> - Link to v1: https://lore.kernel.org/r/20231221-async-free-v1-1-94b277992cb0@kernel.org
> ---
> include/linux/swap_slots.h | 1 +
> mm/swap_slots.c | 20 ++++++++++++++++++++
> 2 files changed, 21 insertions(+)
>
> 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..23dc04bce9ca 100644
> --- a/mm/swap_slots.c
> +++ b/mm/swap_slots.c
> @@ -44,6 +44,7 @@ static DEFINE_MUTEX(swap_slots_cache_mutex);
> static DEFINE_MUTEX(swap_slots_cache_enable_mutex);
>
> 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 +150,7 @@ 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;
> @@ -269,12 +271,27 @@ 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 && cache->n_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;
>
> cache = raw_cpu_ptr(&swp_slots);
> if (likely(use_swap_slot_cache && cache->slots_ret)) {
> + bool full;
> spin_lock_irq(&cache->free_lock);
> /* Swap slots cache may be deactivated before acquiring lock */
> if (!use_swap_slot_cache || !cache->slots_ret) {
> @@ -292,7 +309,10 @@ void free_swap_slot(swp_entry_t entry)
> cache->n_ret = 0;
> }
> cache->slots_ret[cache->n_ret++] = entry;
> + full = cache->n_ret >= SWAP_SLOTS_CACHE_SIZE;
> spin_unlock_irq(&cache->free_lock);
> + if (full)
> + schedule_work(&cache->async_free);
> } else {
> direct_free:
> swapcache_free_entries(&entry, 1);
>
> ---
> base-commit: eacce8189e28717da6f44ee492b7404c636ae0de
> change-id: 20231216-async-free-bef392015432
>
> Best regards,
Powered by blists - more mailing lists