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Subject[RFC] percpu: decrease pcpu_nr_slots by 1
Date
Entry pcpu_slot[pcpu_nr_slots - 2] is wasted with current code logic.
pcpu_nr_slots is calculated with `__pcpu_size_to_slot(size) + 2`.
Take pcpu_unit_size as 1024 for example, __pcpu_size_to_slot will
return max(11 - PCPU_SLOT_BASE_SHIFT + 2, 1), it is 8, so the
pcpu_nr_slots will be 10.

The chunk with free_bytes 1024 will be linked into pcpu_slot[9].
However free_bytes in range [512,1024) will be linked into
pcpu_slot[7], because `fls(512) - PCPU_SLOT_BASE_SHIFT + 2` is 7.
So pcpu_slot[8] is has no chance to be used.

According comments of PCPU_SLOT_BASE_SHIFT, 1~31 bytes share the same slot
and PCPU_SLOT_BASE_SHIFT is defined as 5. But actually 1~15 share the
same slot 1 if we not take PCPU_MIN_ALLOC_SIZE into consideration, 16~31
share slot 2. Calculation as below:
highbit = fls(16) -> highbit = 5
max(5 - PCPU_SLOT_BASE_SHIFT + 2, 1) equals 2, not 1.

This patch by decreasing pcpu_nr_slots to avoid waste one slot and
let [PCPU_MIN_ALLOC_SIZE, 31) really share the same slot.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
---

V1:
Not very sure about whether it is intended to leave the slot there.

mm/percpu.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 8d9933db6162..12a9ba38f0b5 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -219,7 +219,7 @@ static bool pcpu_addr_in_chunk(struct pcpu_chunk *chunk, void *addr)
static int __pcpu_size_to_slot(int size)
{
int highbit = fls(size); /* size is in bytes */
- return max(highbit - PCPU_SLOT_BASE_SHIFT + 2, 1);
+ return max(highbit - PCPU_SLOT_BASE_SHIFT + 1, 1);
}

static int pcpu_size_to_slot(int size)
@@ -2145,7 +2145,7 @@ int __init pcpu_setup_first_chunk(const struct pcpu_alloc_info *ai,
* Allocate chunk slots. The additional last slot is for
* empty chunks.
*/
- pcpu_nr_slots = __pcpu_size_to_slot(pcpu_unit_size) + 2;
+ pcpu_nr_slots = __pcpu_size_to_slot(pcpu_unit_size) + 1;
pcpu_slot = memblock_alloc(pcpu_nr_slots * sizeof(pcpu_slot[0]),
SMP_CACHE_BYTES);
for (i = 0; i < pcpu_nr_slots; i++)
--
2.16.4
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