Re: [PATCH v2 20/23] percpu: update free path to take advantage of contig hints

[Josef Bacik]

On Mon, Jul 24, 2017 at 07:02:17PM -0400, Dennis Zhou wrote:
> From: "Dennis Zhou (Facebook)" <dennisszhou@xxxxxxxxx>
> 
> The bitmap allocator must keep metadata consistent. The easiest way is
> to scan after every allocation for each affected block and the entire
> chunk. This is rather expensive.
> 
> The free path can take advantage of current contig hints to prevent
> scanning within the start and end block.  If a scan is needed, it can
> be done by scanning backwards from the start and forwards from the end
> to identify the entire free area this can be combined with. The blocks
> can then be updated by some basic checks rather than complete block
> scans.
> 
> A chunk scan happens when the freed area makes a page free, a block
> free, or spans across blocks. This is necessary as the contig hint at
> this point could span across blocks. The check uses the minimum of page
> size and the block size to allow for variable sized blocks. There is a
> tradeoff here with not updating after every free. It is possible a
> contig hint in one block can be merged with the contig hint in the next
> block. This means the contig hint can be off by up to a page. However,
> if the chunk's contig hint is contained in one block, the contig hint
> will be accurate.
> 
> Signed-off-by: Dennis Zhou <dennisszhou@xxxxxxxxx>

Reviewed-by: Josef Bacik <jbacik@xxxxxx>

Thanks,

Josef