Ryan Roberts <ryan.roberts@xxxxxxx> writes: > struct percpu_cluster stores the index of cpu's current cluster and the > offset of the next entry that will be allocated for the cpu. These two > pieces of information are redundant because the cluster index is just > (offset / SWAPFILE_CLUSTER). The only reason for explicitly keeping the > cluster index is because the structure used for it also has a flag to > indicate "no cluster". However this data structure also contains a spin > lock, which is never used in this context, as a side effect the code > copies the spinlock_t structure, which is questionable coding practice > in my view. > > So let's clean this up and store only the next offset, and use a > sentinal value (SWAP_NEXT_INVALID) to indicate "no cluster". > SWAP_NEXT_INVALID is chosen to be 0, because 0 will never be seen > legitimately; The first page in the swap file is the swap header, which > is always marked bad to prevent it from being allocated as an entry. > This also prevents the cluster to which it belongs being marked free, so > it will never appear on the free list. > > This change saves 16 bytes per cpu. And given we are shortly going to > extend this mechanism to be per-cpu-AND-per-order, we will end up saving > 16 * 9 = 144 bytes per cpu, which adds up if you have 256 cpus in the > system. > > Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx> LGTM, Thanks! -- Best Regards, Huang, Ying
