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@xxxxxxxxxx> Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> > --- > 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@xxxxxxxxxx > > 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@xxxxxxxxxx > > 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@xxxxxxxxxx > --- > 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,
