Re: [RFC] percpu: decrease pcpu_nr_slots by 1

[dennis@xxxxxxxxxx]

On Sun, Feb 24, 2019 at 09:17:08AM +0000, Peng Fan wrote:
> 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@xxxxxxx>
> ---
> 
> 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);
>  }

Honestly, it may be better to just have [1-16) [16-31) be separate. I'm
working on a change to this area, so I may change what's going on here.

>  
>  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
> 

This is a tricky change. The nice thing about keeping the additional
slot around is that it ensures a distinction between a completely empty
chunk and a nearly empty chunk. It happens to be that the logic creates
power of 2 chunks which ends up being an additional slot anyway. So,
given that this logic is tricky and architecture dependent, I don't feel
comfortable making this change as the risk greatly outweighs the
benefit.

Thanks,
Dennis