Re: [linus:master] [mm] f95bdb700b: stress-ng.ramfs.ops_per_sec -88.8% regression

[Qi Zheng <qi.zheng@linux.dev>]

On 2023/5/24 19:56, Qi Zheng wrote:
> 
> 
> 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:

Or just set rcu_expedited to 1:
	echo 1 > /sys/kernel/rcu_expedited

> 
> 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