Re: [PATCH v2 5/7] mm, swap: use percpu cluster as allocation fast path

[Baoquan He <bhe@redhat.com>]

On 02/25/25 at 02:02am, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> Current allocation workflow first traverses the plist with a global lock
> held, after choosing a device, it uses the percpu cluster on that swap
> device. This commit moves the percpu cluster variable out of being tied
> to individual swap devices, making it a global percpu variable, and will
> be used directly for allocation as a fast path.
> 
> The global percpu cluster variable will never point to a HDD device, and
> allocations on a HDD device are still globally serialized.
> 
> This improves the allocator performance and prepares for removal of the
> slot cache in later commits. There shouldn't be much observable behavior
> change, except one thing: this changes how swap device allocation
> rotation works.
> 
> Currently, each allocation will rotate the plist, and because of the
> existence of slot cache (one order 0 allocation usually returns 64
> entries), swap devices of the same priority are rotated for every 64
> order 0 entries consumed. High order allocations are different, they
> will bypass the slot cache, and so swap device is rotated for every
> 16K, 32K, or up to 2M allocation.
> 
> The rotation rule was never clearly defined or documented, it was changed
> several times without mentioning.
> 
> After this commit, and once slot cache is gone in later commits, swap
> device rotation will happen for every consumed cluster. Ideally non-HDD
> devices will be rotated if 2M space has been consumed for each order.
> Fragmented clusters will rotate the device faster, which seems OK.
> HDD devices is rotated for every allocation regardless of the allocation
> order, which should be OK too and trivial.
> 
> This commit also slightly changes allocation behaviour for slot cache.
> The new added cluster allocation fast path may allocate entries from
> different device to the slot cache, this is not observable from user
> space, only impact performance very slightly, and slot cache will be
> just gone in next commit, so this can be ignored.
> 
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
>  include/linux/swap.h |  11 ++--
>  mm/swapfile.c        | 136 +++++++++++++++++++++++++++++--------------
>  2 files changed, 95 insertions(+), 52 deletions(-)

Reviewed-by: Baoquan He <bhe@redhat.com>