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SubjectHigh IO load cause kswapd hang and unresponsive system
Hi everyone,

Recently when I upgrade my kernel from 3.12.61 to kernel 4.4.74, I
find my system sometimes is very slow and unresponsive

When my system is under a heavy IO load, I see the node start to
respond very slow, the IO wait time keeps very high, and the node
starts unresponsive. I try to dump all block task at that time and
found some interesting things, look like under a heavy load I/O, the
kswap start to hang in jbd2_log_wait_commit.

Sep 07 12:25:35 myPC kernel: [11039.126514] kswapd0 D
0000000000000000 0 26 2 0x00000000
Sep 07 12:25:35 myPC kernel: [11039.126517] ffff88003da27b78
ffff88000ce08280 ffff88003cdf0680 ffff88003da28000
Sep 07 12:25:35 myPC kernel: [11039.126519] ffff88001a15c888
ffff88001a15c824 ffff88003da27bb8 ffff88001a15c8a0
Sep 07 12:25:35 myPC kernel: [11039.126521] ffff88003da27b90
ffffffff815e4c85 ffff88001a15c800 0000000000013183
Sep 07 12:25:35 myPC kernel: [11039.126522] Call Trace:
Sep 07 12:25:35 myPC kernel: [11039.126539] [<ffffffff815e4c85>]
schedule+0x35/0x80
Sep 07 12:25:35 myPC kernel: [11039.126561] [<ffffffffa02faada>]
jbd2_log_wait_commit+0x8a/0xf0 [jbd2]
Sep 07 12:25:35 myPC kernel: [11039.126621] [<ffffffffa03b28a4>]
ext4_evict_inode+0x2d4/0x4b0 [ext4]
Sep 07 12:25:35 myPC kernel: [11039.126630] [<ffffffff81215a7a>]
evict+0xba/0x190
Sep 07 12:25:35 myPC kernel: [11039.126635] [<ffffffff81215b82>]
dispose_list+0x32/0x50
Sep 07 12:25:35 myPC kernel: [11039.126639] [<ffffffff81216e52>]
prune_icache_sb+0x42/0x50
Sep 07 12:25:35 myPC kernel: [11039.126644] [<ffffffff811ff8f6>]
super_cache_scan+0x136/0x180
Sep 07 12:25:35 myPC kernel: [11039.126653] [<ffffffff81196a1b>]
shrink_slab.part.41+0x20b/0x3f0
Sep 07 12:25:35 myPC kernel: [11039.126521] ffff88003da27b90
ffffffff815e4c85 ffff88001a15c800 0000000000013183

And after that more and more processes are hitting STAT D, nfsd is
hang to wait or the inode deletion to complete in
__wait_on_freeing_inode.

Sep 07 12:25:35 myPC kernel: [11039.390218] nfsd D
0000000000000000 0 30419 2 0x00000080
Sep 07 12:25:35 myPC kernel: [11039.390221] ffff88000da47a98
ffff8800277ec380 ffff8800374ec580 ffff88000da48000
Sep 07 12:25:35 myPC kernel: [11039.390223] ffff88000da47ad8
ffffc9000011edc8 0000000000000000 ffff88000e92e2f8
Sep 07 12:25:35 myPC kernel: [11039.390225] ffff88000da47ab0
ffffffff815e4c85 ffffffff81c094c0 ffff88003ff9d8b8
Sep 07 12:25:35 myPC kernel: [11039.390225] Call Trace:
Sep 07 12:25:35 myPC kernel: [11039.390231] [<ffffffff815e4c85>]
schedule+0x35/0x80
Sep 07 12:25:35 myPC kernel: [11039.390236] [<ffffffff812157fc>]
__wait_on_freeing_inode+0x9c/0xc0
Sep 07 12:25:35 myPC kernel: [11039.390241] [<ffffffff81215d83>]
find_inode_fast+0x73/0xa0
Sep 07 12:25:35 myPC kernel: [11039.390245] [<ffffffff81216052>]
iget_locked+0x72/0x190
Sep 07 12:25:35 myPC kernel: [11039.390284] [<ffffffffa03aeb8c>]
ext4_iget+0x2c/0xb10 [ext4]
Sep 07 12:25:35 myPC kernel: [11039.390314] [<ffffffffa03c1055>]
ext4_nfs_get_inode+0x45/0x80 [ext4]
Sep 07 12:25:35 myPC kernel: [11039.390320] [<ffffffff812211c4>]
generic_fh_to_dentry+0x24/0x30
Sep 07 12:25:35 myPC kernel: [11039.390326] [<ffffffff81277ede>]
exportfs_decode_fh+0x5e/0x350
Sep 07 12:25:35 myPC kernel: [11039.390347] [<ffffffffa047749a>]
fh_verify+0x2da/0x5a0 [nfsd]
Sep 07 12:25:35 myPC kernel: [11039.390362] [<ffffffffa0480933>]
nfsd3_proc_getattr+0x63/0xd0 [nfsd]
Sep 07 12:25:35 myPC kernel: [11039.390374] [<ffffffffa0473e23>]
nfsd_dispatch+0xc3/0x260 [nfsd]
Sep 07 12:25:35 myPC kernel: [11039.390411] [<ffffffffa0322108>]
svc_process_common+0x418/0x6a0 [sunrpc]
Sep 07 12:25:35 myPC kernel: [11039.390439] [<ffffffffa032248d>]
svc_process+0xfd/0x1b0 [sunrpc]
Sep 07 12:25:35 myPC kernel: [11039.390454] [<ffffffffa047389a>]
nfsd+0xea/0x160 [nfsd]

That causes the other nfs client is hang to wait nfsd to continue.

Sep 07 12:25:35 myPC kernel: [11039.207987] Call Trace:
Sep 07 12:25:35 myPC kernel: [11039.207994] [<ffffffff815e4c85>]
schedule+0x35/0x80
Sep 07 12:25:35 myPC kernel: [11039.208008] [<ffffffffa031acce>]
rpc_wait_bit_killable+0x1e/0xa0 [sunrpc]
Sep 07 12:25:35 myPC kernel: [11039.208055] [<ffffffff815e5098>]
__wait_on_bit+0x58/0x90
Sep 07 12:25:35 myPC kernel: [11039.208059] [<ffffffff815e513e>]
out_of_line_wait_on_bit+0x6e/0x80
Sep 07 12:25:35 myPC kernel: [11039.208082] [<ffffffffa031bd26>]
__rpc_execute+0x146/0x440 [sunrpc]
Sep 07 12:25:35 myPC kernel: [11039.208104] [<ffffffffa0313049>]
rpc_run_task+0x69/0x80 [sunrpc]
Sep 07 12:25:35 myPC kernel: [11039.208126] [<ffffffffa031309f>]
rpc_call_sync+0x3f/0xa0 [sunrpc]
Sep 07 12:25:35 myPC kernel: [11039.204818] [<ffffffff81200d69>]
vfs_fstatat+0x49/0x90
Sep 07 12:25:35 myPC kernel: [11039.204821] [<ffffffff812011ea>]
SYSC_newstat+0x1a/0x40
Sep 07 12:25:35 myPC kernel: [11039.204826] [<ffffffff815e876e>]
entry_SYSCALL_64_fastpath+0x12/0x6d
Sep 07 12:25:35 myPC kernel: [11039.207972] DWARF2 unwinder stuck at
entry_SYSCALL_64_fastpath+0x12/0x6d

I guess under heavy load, somehow the inode cannot be evicted in
kswap, so the nfsd just keeps waiting and cause the whole system ends
up in unresponsive state. I found some related issues
https://www.systutorials.com/linux-kernels/105429/nfsv4-1-prevent-a-3-way-deadlock-between-layoutreturn-open-and-state-recovery-linux-3-12-17/
(it’s not the same as it’s nfsv4 but just refer for the same symptom.)

My system has very limited resource, only 1 core and 1GB RAM and the
issue is very easy to happen in a slow disk. However, I did not see
the same issue in the older kernel 3.12.61. Could you please let me
know if my analysis makes sense, if jbd2_log_wait_commit hang causes
__wait_on_freeing_inode wait ? Is it a known issue in kernel? If any
kernel change causes the issue?. And if we have any WA to avoid that
like preventing kswapd kicks in (I have a 5G swap partition) or to
make my system is not unresponsive like that (of course it is not to
add more RAM :) ?

Thanks a lot
Brs,
Naruto

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