bcache: fixup bcache_dev_sectors_dirty_add() multithreaded CPU false sharing

When attaching a cached device (a.k.a backing device) to a cache
device, bch_sectors_dirty_init() is called to count dirty sectors
and stripes (see what bcache_dev_sectors_dirty_add() does) on the
cache device.

When bcache_dev_sectors_dirty_add() is called, set_bit(stripe,
d->full_dirty_stripes) or clear_bit(stripe, d->full_dirty_stripes)
operation will always be performed. In full_dirty_stripes, each 1bit
represents stripe_size (8192) sectors (512B), so 1bit=4MB (8192*512),
and each CPU cache line=64B=512bit=2048MB. When 20 threads process
a cached disk with 100G dirty data, a single thread processes about
23M at a time, and 20 threads total 460M. These full_dirty_stripes
bits corresponding to the 460M data is likely to fall in the same CPU
cache line. When one of these threads performs a set_bit or clear_bit
operation, the same CPU cache line of other threads will become invalid
and must read the full_dirty_stripes from the main memory again. Compared
with single thread, the time of a bcache_dev_sectors_dirty_add()
call is increased by about 50 times in our test (100G dirty data,
20 threads, bcache_dev_sectors_dirty_add() is called more than
20 million times).

This patch tries to test_bit before set_bit or clear_bit operation.
Therefore, a lot of force set and clear operations will be avoided,
and most of bcache_dev_sectors_dirty_add() calls will only read CPU
cache line.

Signed-off-by: Mingzhe Zou <mingzhe.zou@easystack.cn>
Signed-off-by: Coly Li <colyli@suse.de>

diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index d42301e6309d4b287863aff7b9cab0e441618d27..176461f89f46f84d342735c5fa6b1e607b0038f9 100644 (file)
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -585,10 +585,13 @@ void bcache_dev_sectors_dirty_add(struct cache_set *c, unsigned int inode,
 
                sectors_dirty = atomic_add_return(s,
                                        d->stripe_sectors_dirty + stripe);
-               if (sectors_dirty == d->stripe_size)
-                       set_bit(stripe, d->full_dirty_stripes);
-               else
-                       clear_bit(stripe, d->full_dirty_stripes);
+               if (sectors_dirty == d->stripe_size) {
+                       if (!test_bit(stripe, d->full_dirty_stripes))
+                               set_bit(stripe, d->full_dirty_stripes);
+               } else {
+                       if (test_bit(stripe, d->full_dirty_stripes))
+                               clear_bit(stripe, d->full_dirty_stripes);
+               }
 
                nr_sectors -= s;
                stripe_offset = 0;