Re: [PATCH v2 04/19] btrfs: remove the open-code to read disk-key

David Sterba
2020-09-17T12:54:27Z

On Thu, Sep 17, 2020 at 04:02:37PM +0800, Qu Wenruo wrote:
> On 2020/9/17 上午12:01, David Sterba wrote:
> > On Tue, Sep 15, 2020 at 01:35:17PM +0800, Qu Wenruo wrote:
> >> generic_bin_search() distinguishes between reading a key which doesn't
> >> cross a page and one which does. However this distinction is not
> >> necessary since read_extent_buffer handles both cases transparently.
> >>
> >> Just use read_extent_buffer to streamline the code.
> >>
> >> Signed-off-by: Qu Wenruo <wqu@suse.com>
> >> ---
> >> fs/btrfs/ctree.c | 13 ++-----------
> >> 1 file changed, 2 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> >> index cd1cd673bc0b..e204e1320745 100644
> >> --- a/fs/btrfs/ctree.c
> >> +++ b/fs/btrfs/ctree.c
> >> @@ -1697,7 +1697,6 @@ static noinline int generic_bin_search(struct extent_buffer *eb,
> >> }
> >>
> >> while (low < high) {
> >> - unsigned long oip;
> >> unsigned long offset;
> >> struct btrfs_disk_key *tmp;
> >> struct btrfs_disk_key unaligned;
> >> @@ -1705,17 +1704,9 @@ static noinline int generic_bin_search(struct extent_buffer *eb,
> >>
> >> mid = (low + high) / 2;
> >> offset = p + mid * item_size;
> >> - oip = offset_in_page(offset);
> >>
> >> - if (oip + key_size <= PAGE_SIZE) {
> >> - const unsigned long idx = offset >> PAGE_SHIFT;
> >> - char *kaddr = page_address(eb->pages[idx]);
> >> -
> >> - tmp = (struct btrfs_disk_key *)(kaddr + oip);
> >> - } else {
> >> - read_extent_buffer(eb, &unaligned, offset, key_size);
> >> - tmp = &unaligned;
> >> - }
> >> + read_extent_buffer(eb, &unaligned, offset, key_size);
> >> + tmp = &unaligned;
> >
> > Reading from the first page is a performance optimization on systems
> > with 4K pages, ie. the majority. I'm not in favor removing it just to
> > make the code look nicer.
>
> For 4K system, with the optimization it only saves one
> read_extent_buffer() call cost.

This evaluation is wrong, you missed several things that
generic_bin_search and read_extent_buffer do.

generic_bin_search is called very often, each search slot so
optimization is worth here

read_extent_buffer is used _only_ for keys that cross page boundary, so
we need to read the bytes in two steps and this is wrapped into a
function that we call in a limited number of cases

In all other cases, when the whole key is contained in the page the call
is inline in generic_bin_search, ie. no function call overhead

> Or we will need to manually call get_eb_page_offset() here to make it
> work for subpage.

For nodesize that is smaller than PAGE_SIZE there's no page crossing at
all so using read_extent_buffer would be making things worse.