From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Hi all,
This patch series aims to make unregistration of super_block shrinker more
faster.
1. Background
=============
The kernel test robot noticed a -88.8% regression of stress-ng.ramfs.ops_per_sec
on commit f95bdb700bc6 ("mm: vmscan: make global slab shrink lockless"). More
details can be seen from the link[1] below.
[1]. https://lore.kernel.org/lkml/202305230837.db2c233f-yujie.liu@xxxxxxxxx/
We can just use the following command to reproduce the result:
stress-ng --timeout 60 --times --verify --metrics-brief --ramfs 9 &
1) before commit f95bdb700bc6b:
stress-ng: info: [11023] dispatching hogs: 9 ramfs
stress-ng: info: [11023] stressor bogo ops real time usr time sys time bogo ops/s bogo ops/s
stress-ng: info: [11023] (secs) (secs) (secs) (real time) (usr+sys time)
stress-ng: info: [11023] ramfs 774966 60.00 10.18 169.45 12915.89 4314.26
stress-ng: info: [11023] for a 60.00s run time:
stress-ng: info: [11023] 1920.11s available CPU time
stress-ng: info: [11023] 10.18s user time ( 0.53%)
stress-ng: info: [11023] 169.44s system time ( 8.82%)
stress-ng: info: [11023] 179.62s total time ( 9.35%)
stress-ng: info: [11023] load average: 8.99 2.69 0.93
stress-ng: info: [11023] successful run completed in 60.00s (1 min, 0.00 secs)
2) after commit f95bdb700bc6b:
stress-ng: info: [37676] dispatching hogs: 9 ramfs
stress-ng: info: [37676] stressor bogo ops real time usrtime sys time bogo ops/s bogo ops/s
stress-ng: info: [37676] (secs) (secs) (secs) (real time) (usr+sys time)
stress-ng: info: [37676] ramfs 168673 60.00 1.61 39.66 2811.08 4087.47
stress-ng: info: [37676] for a 60.10s run time:
stress-ng: info: [37676] 1923.36s available CPU time
stress-ng: info: [37676] 1.60s user time ( 0.08%)
stress-ng: info: [37676] 39.66s system time ( 2.06%)
stress-ng: info: [37676] 41.26s total time ( 2.15%)
stress-ng: info: [37676] load average: 7.69 3.63 2.36
stress-ng: info: [37676] successful run completed in 60.10s (1 min, 0.10 secs)
The root cause is that SRCU has to be careful to not frequently check for srcu
read-side critical section exits. Paul E. McKenney gave a detailed explanation:
```
In practice, the act of checking to see if there is anyone in an SRCU
read-side critical section is a heavy-weight operation, involving at
least one cache miss per CPU along with a number of full memory barriers.
```
Therefore, even if no one is currently in the SRCU read-side critical section,
synchronize_srcu() cannot return quickly. That's why unregister_shrinker() has
become slower.
2. Idea
=======
2.1 use synchronize_srcu_expedited() ?
--------------------------------------
The synchronize_srcu_expedited() will let SRCU to be much more aggressive.
If we use it to replace synchronize_srcu() in the unregister_shrinker(), the
ops/s will return to previous levels:
stress-ng: info: [13159] dispatching hogs: 9 ramfs
stress-ng: info: [13159] stressor bogo ops real time usr time sys time bogo ops/s bogo ops/s
stress-ng: info: [13159] (secs) (secs) (secs) (real time) (usr+sys time)
stress-ng: info: [13159] ramfs 710062 60.00 9.63 157.26 11834.18 4254.75
stress-ng: info: [13159] for a 60.00s run time:
stress-ng: info: [13159] 1920.14s available CPU time
stress-ng: info: [13159] 9.62s user time ( 0.50%)
stress-ng: info: [13159] 157.26s system time ( 8.19%)
stress-ng: info: [13159] 166.88s total time ( 8.69%)
stress-ng: info: [13159] load average: 9.49 4.02 1.65
stress-ng: info: [13159] successful run completed in 60.00s (1 min, 0.00 secs)
But because SRCU (Sleepable RCU) is used here, the reader is allowed to sleep in
the read-side critical section, so synchronize_srcu_expedited() may cause a lot
of CPU consumption, so this is not a good choice.
2.2 move synchronize_srcu() to the asynchronous delayed work
------------------------------------------------------------
Kirill Tkhai proposed a better idea[2] in 2018: move synchronize_srcu() to the
asynchronous delayed work, then it doesn't affect on user-visible unregistration
speed.
[2]. https://lore.kernel.org/lkml/153365636747.19074.12610817307548583381.stgit@localhost.localdomain/
After applying his patches ([PATCH RFC 04/10]~[PATCH RFC 10/10], with few
conflicts), the ops/s is of course back to the previous levels:
stress-ng: info: [11506] setting to a 60 second run per stressor
stress-ng: info: [11506] dispatching hogs: 9 ramfs
stress-ng: info: [11506] stressor bogo ops real time usr time sys time bogo ops/s bogo ops/s
stress-ng: info: [11506] (secs) (secs) (secs) (real time) (usr+sys time)
stress-ng: info: [11506] ramfs 829462 60.00 10.81 174.25 13824.14 4482.08
stress-ng: info: [11506] for a 60.00s run time:
stress-ng: info: [11506] 1920.12s available CPU time
stress-ng: info: [11506] 10.81s user time ( 0.56%)
stress-ng: info: [11506] 174.25s system time ( 9.07%)
stress-ng: info: [11506] 185.06s total time ( 9.64%)
stress-ng: info: [11506] load average: 8.96 2.60 0.89
stress-ng: info: [11506] successful run completed in 60.00s (1 min, 0.00 secs)
In order to continue to advance this patch set, I rebase these patches onto the
next-20230525. Any comments and suggestions are welcome.
Note: This patch serise is only for super_block shrinker, all further
time-critical for unregistration places may be written in the same conception.
Thanks,
Qi
Kirill Tkhai (7):
mm: vmscan: split unregister_shrinker()
fs: move list_lru_destroy() to destroy_super_work()
fs: shrink only (SB_ACTIVE|SB_BORN) superblocks in super_cache_scan()
fs: introduce struct super_operations::destroy_super() callback
xfs: introduce xfs_fs_destroy_super()
shmem: implement shmem_destroy_super()
fs: use unregister_shrinker_delayed_{initiate, finalize} for
super_block shrinker
Qi Zheng (1):
mm: vmscan: move shrinker_debugfs_remove() before synchronize_srcu()
fs/super.c | 32 ++++++++++++++++++--------------
fs/xfs/xfs_super.c | 25 ++++++++++++++++++++++---
include/linux/fs.h | 6 ++++++
include/linux/shrinker.h | 2 ++
mm/shmem.c | 8 ++++++++
mm/vmscan.c | 26 ++++++++++++++++++++------
6 files changed, 76 insertions(+), 23 deletions(-)
--
2.30.2
