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

["Huang\, Ying" <ying.huang@intel.com>]

Hi, Rik,

Thanks for comments!

Rik van Riel <riel@redhat.com> writes:
> 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.

Asked Dan and Ross for DAX usage of writeback tags.  The DAX uses these
tags to flush the cache, etc.

>From Dan:

"
DAX uses them to track PMEM ranges that have taken a write fault so
that we can flush/write-back those dirty ranges at fsync()/msync()
time.
"

But I still think that it may be a good idea to use some function or
flags for this.  Because it is more flexible and readable.

Best Regards,
Huang, Ying