Re: [PATCH v3] mm: swap: async free swap slot cache entries

[Tim Chen]

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

>