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Message-ID: <be04dc3e-a671-ec70-6cf6-70dc702f4184@linux.dev>
Date: Thu, 25 May 2023 12:03:16 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: RCU <rcu@...r.kernel.org>, Yujie Liu <yujie.liu@...el.com>
Cc: oe-lkp@...ts.linux.dev, lkp@...el.com,
linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Vlastimil Babka <vbabka@...e.cz>, Kirill Tkhai <tkhai@...ru>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Christian König <christian.koenig@....com>,
David Hildenbrand <david@...hat.com>,
Davidlohr Bueso <dave@...olabs.net>,
Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...nel.org>,
Muchun Song <muchun.song@...ux.dev>,
"Paul E. McKenney" <paulmck@...nel.org>,
Shakeel Butt <shakeelb@...gle.com>,
Yang Shi <shy828301@...il.com>, linux-mm@...ck.org,
ying.huang@...el.com, feng.tang@...el.com, fengwei.yin@...el.com
Subject: Re: [linus:master] [mm] f95bdb700b: stress-ng.ramfs.ops_per_sec
-88.8% regression
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
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