On Tue, 2024-02-13 at 15:20 -0800, Chris Li 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. > > Add /sys/kernel/mm/swap/swap_slot_async_free to control the > async free behavior. When enabled, using work queue to async > free the swap slot when the swap slot cache is full. > > 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> > --- > 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 | 2 ++ > mm/swap_slots.c | 20 ++++++++++++++++++++ > mm/swap_state.c | 23 +++++++++++++++++++++++ > 3 files changed, 45 insertions(+) > > diff --git a/include/linux/swap_slots.h b/include/linux/swap_slots.h > index 15adfb8c813a..bb9a401d7cae 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); > @@ -27,5 +28,6 @@ void enable_swap_slots_cache(void); > void free_swap_slot(swp_entry_t entry); > > extern bool swap_slot_cache_enabled; > +extern uint8_t slot_cache_async_free __read_mostly; Why wouldn't you enable the async_free always? Otherwise the patch looks fine to me. Tim >
