Re: [PATCH] mm: Don't use radix tree writeback tags for pages in swap cache

[Rik van Riel <riel@redhat.com>]

On Thu, 2016-08-25 at 12:27 -0700, Huang, Ying wrote:
> File pages use a set of radix tags (DIRTY, TOWRITE, WRITEBACK, etc.)
> to
> accelerate finding the pages with a specific tag in the radix tree
> during inode writeback.  But for anonymous pages in the swap cache,
> there is no inode writeback.  So there is no need to find the
> pages with some writeback tags in the radix tree.  It is not
> necessary
> to touch radix tree writeback tags for pages in the swap cache.
> 
> With this patch, the swap out bandwidth improved 22.3% (from ~1.2GB/s
> to
> ~ 1.48GBps) in the vm-scalability swap-w-seq test case with 8
> processes.
> The test is done on a Xeon E5 v3 system.  The swap device used is a
> RAM
> simulated PMEM (persistent memory) device.  The improvement comes
> from
> the reduced contention on the swap cache radix tree lock.  To test
> sequential swapping out, the test case uses 8 processes, which
> sequentially allocate and write to the anonymous pages until RAM and
> part of the swap device is used up.
> 
> Details of comparison is as follow,
> 
> base             base+patch
> ---------------- --------------------------
>          %stddev     %change         %stddev
>              \          |                \
>    1207402 ±  7%     +22.3%    1476578 ±  6%  vmstat.swap.so
>    2506952 ±  2%     +28.1%    3212076 ±  7%  vm-
> scalability.throughput
>      10.86 ± 12%     -23.4%       8.31 ± 16%  perf-profile.cycles-
> pp._raw_spin_lock_irq.__add_to_swap_cache.add_to_swap_cache.add_to_sw
> ap.shrink_page_list
>      10.82 ± 13%     -33.1%       7.24 ± 14%  perf-profile.cycles-
> pp._raw_spin_lock_irqsave.__remove_mapping.shrink_page_list.shrink_in
> active_list.shrink_zone_memcg
>      10.36 ± 11%    -100.0%       0.00 ± -1%  perf-profile.cycles-
> pp._raw_spin_lock_irqsave.__test_set_page_writeback.bdev_write_page._
> _swap_writepage.swap_writepage
>      10.52 ± 12%    -100.0%       0.00 ± -1%  perf-profile.cycles-
> pp._raw_spin_lock_irqsave.test_clear_page_writeback.end_page_writebac
> k.page_endio.pmem_rw_page
> 
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Shaohua Li <shli@kernel.org>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Wu Fengguang <fengguang.wu@intel.com>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> ---
>  mm/page-writeback.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 82e7252..599d2f9 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2728,7 +2728,8 @@ int test_clear_page_writeback(struct page
> *page)
>  	int ret;
>  
>  	lock_page_memcg(page);
> -	if (mapping) {
> +	/* Pages in swap cache don't use writeback tags */
> +	if (mapping && !PageSwapCache(page)) {

I wonder if that should be a mapping_uses_tags(mapping)
macro or similar, and a per-mapping flag?

I suspect there will be another case coming up soon
where we have a page cache radix tree, but no need
for dirty/writeback/... tags.

That use case would be DAX filesystems, where we do
use a struct page, but that struct page points at
persistent storage, and the tags are not necessary.