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authorMingzhe Zou <mingzhe.zou@easystack.cn>2022-01-07 16:21:13 +0800
committerColy Li <colyli@suse.de>2022-03-06 22:33:37 +0800
commit7b1002f7cfe581930f63787a0b3de0144e61ed55 (patch)
treeb348bdf5de00503d4ca530cce6880dd15b88832d /drivers/md
parent13d4ef0f66b7ee9415e101e213acaf94a0cb28ee (diff)
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>
Diffstat (limited to 'drivers/md')
-rw-r--r--drivers/md/bcache/writeback.c11
1 files changed, 7 insertions, 4 deletions
diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index d42301e6309d..176461f89f46 100644
--- 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;