Re: [linus:master] [mm] f95bdb700b: stress-ng.ramfs.ops_per_sec -88.8% regression
From: Qi Zheng
Date: Wed May 24 2023 - 07:56:51 EST
On 2023/5/24 19:08, Qi Zheng wrote:
[...]
Well, I just ran the following command and reproduced the result:
stress-ng --timeout 60 --times --verify --metrics-brief --ramfs 9 &
1) with commit 42c9db3970483:
stress-ng: info: [11023] setting to a 60 second run per stressor
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) with commit f95bdb700bc6b:
stress-ng: info: [37676] dispatching hogs: 9 ramfs
stress-ng: info: [37676] stressor bogo ops real time usr time
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 bogo ops/s (real time) did drop significantly.
And the memory reclaimation was not triggered in the whole process. so
theoretically no one is in the read critical section of shrinker_srcu.
Then I found that some stress-ng-ramfs processes were in
TASK_UNINTERRUPTIBLE state for a long time:
root 42313 0.0 0.0 69592 2068 pts/0 S 19:00 0:00
stress-ng-ramfs [run]
root 42314 0.0 0.0 69592 2068 pts/0 S 19:00 0:00
stress-ng-ramfs [run]
root 42315 0.0 0.0 69592 2068 pts/0 S 19:00 0:00
stress-ng-ramfs [run]
root 42316 0.0 0.0 69592 2068 pts/0 S 19:00 0:00
stress-ng-ramfs [run]
root 42317 7.8 0.0 69592 1812 pts/0 D 19:00 0:02
stress-ng-ramfs [run]
root 42318 0.0 0.0 69592 2068 pts/0 S 19:00 0:00
stress-ng-ramfs [run]
root 42319 7.8 0.0 69592 1812 pts/0 D 19:00 0:02
stress-ng-ramfs [run]
root 42320 0.0 0.0 69592 2068 pts/0 S 19:00 0:00
stress-ng-ramfs [run]
root 42321 7.8 0.0 69592 1812 pts/0 D 19:00 0:02
stress-ng-ramfs [run]
root 42322 0.0 0.0 69592 2068 pts/0 S 19:00 0:00
stress-ng-ramfs [run]
root 42323 7.8 0.0 69592 1812 pts/0 D 19:00 0:02
stress-ng-ramfs [run]
root 42324 0.0 0.0 69592 2068 pts/0 S 19:00 0:00
stress-ng-ramfs [run]
root 42325 7.8 0.0 69592 1812 pts/0 D 19:00 0:02
stress-ng-ramfs [run]
root 42326 0.0 0.0 69592 2068 pts/0 S 19:00 0:00
stress-ng-ramfs [run]
root 42327 7.9 0.0 69592 1812 pts/0 D 19:00 0:02
stress-ng-ramfs [run]
root 42328 7.9 0.0 69592 1812 pts/0 D 19:00 0:02
stress-ng-ramfs [run]
root 42329 7.9 0.0 69592 1812 pts/0 D 19:00 0:02
stress-ng-ramfs [run]
root 42330 7.9 0.0 69592 1556 pts/0 D 19:00 0:02
stress-ng-ramfs [run]
Their call stack is as follows:
cat /proc/42330/stack
[<0>] __synchronize_srcu.part.21+0x83/0xb0
[<0>] unregister_shrinker+0x85/0xb0
[<0>] deactivate_locked_super+0x27/0x70
[<0>] cleanup_mnt+0xb8/0x140
[<0>] task_work_run+0x65/0x90
[<0>] exit_to_user_mode_prepare+0x1ba/0x1c0
[<0>] syscall_exit_to_user_mode+0x1b/0x40
[<0>] do_syscall_64+0x44/0x80
[<0>] entry_SYSCALL_64_after_hwframe+0x63/0xcd
+ RCU folks, Is this result as expected? I would have thought that
synchronize_srcu() should return quickly if no one is in the read
critical section. :(
With the following changes, ops/s can return to previous levels:
diff --git a/mm/vmscan.c b/mm/vmscan.c
index db2ed6e08f67..90f541b07cd1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -763,7 +763,7 @@ void unregister_shrinker(struct shrinker *shrinker)
debugfs_entry = shrinker_debugfs_remove(shrinker);
up_write(&shrinker_rwsem);
- synchronize_srcu(&shrinker_srcu);
+ synchronize_srcu_expedited(&shrinker_srcu);
debugfs_remove_recursive(debugfs_entry);
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)
Can we make synchronize_srcu() call synchronize_srcu_expedited() when no
one is in the read critical section?
--
Thanks,
Qi