On Wed, Nov 28, 2018 at 6:18 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>
> On Wed, Nov 28, 2018 at 04:36:25PM -0800, Ivan Babrou wrote:
> > Hello,
> >
> > We're experiencing some interesting issues with memory reclaim, both
> > kswapd and direct reclaim.
> >
> > A typical machine is 2 x NUMA with 128GB of RAM and 6 XFS filesystems.
> > Page cache is around 95GB and dirty pages hover around 50MB, rarely
> > jumping up to 1GB.
>
> What is your workload?
My test setup is an empty machine with 256GB of RAM booted from
network into memory with just systemd essentials running.
I create XFS on a 10TB drive (via LUKS), mount the drive and write
300GiB of randomness:
$ sudo mkfs.xfs /dev/mapper/luks-sda
$ sudo mount /dev/mapper/luks-sda /mnt
$ sudo dd if=/dev/urandom of=/mnt/300g.random bs=1M count=300K status=progress
Then I reboot and just mount the drive again to run my test workload:
$ dd if=/mnt/300g.random of=/dev/null bs=1M status=progress
After running it once and populating page cache I restart it to collect traces.
Here's xfs_info:
$ sudo xfs_info /mnt
meta-data=/dev/mapper/luks-sda isize=512 agcount=10, agsize=268435455 blks
= sectsz=4096 attr=2, projid32bit=1
= crc=1 finobt=1 spinodes=0 rmapbt=0
= reflink=0
data = bsize=4096 blocks=2441608704, imaxpct=5
= sunit=0 swidth=0 blks
naming =version 2 bsize=4096 ascii-ci=0 ftype=1
log =internal bsize=4096 blocks=521728, version=2
= sectsz=4096 sunit=1 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0
And here's slabinfo:
$ sudo cat /proc/slabinfo
slabinfo - version: 2.1
# name <active_objs> <num_objs> <objsize> <objperslab>
<pagesperslab> : tunables <limit> <batchcount> <sharedfactor> :
slabdata <active_slabs> <num_slabs> <sharedavail>
kcopyd_job 0 0 3312 9 8 : tunables 0 0
0 : slabdata 0 0 0
dm_uevent 0 0 2632 12 8 : tunables 0 0
0 : slabdata 0 0 0
dax_cache 96 504 768 42 8 : tunables 0 0
0 : slabdata 12 12 0
nf_conntrack_expect 0 0 224 36 2 : tunables 0
0 0 : slabdata 0 0 0
nf_conntrack 0 0 320 51 4 : tunables 0 0
0 : slabdata 0 0 0
kvm_async_pf 0 0 136 30 1 : tunables 0 0
0 : slabdata 0 0 0
kvm_vcpu 0 0 19392 1 8 : tunables 0 0
0 : slabdata 0 0 0
kvm_mmu_page_header 0 0 168 48 2 : tunables 0
0 0 : slabdata 0 0 0
pte_list_desc 0 0 32 128 1 : tunables 0 0
0 : slabdata 0 0 0
ip6-frags 0 0 208 39 2 : tunables 0 0
0 : slabdata 0 0 0
RAWv6 168 168 1152 28 8 : tunables 0 0
0 : slabdata 6 6 0
UDPv6 900 900 1280 25 8 : tunables 0 0
0 : slabdata 36 36 0
tw_sock_TCPv6 1122 1122 248 33 2 : tunables 0 0
0 : slabdata 34 34 0
request_sock_TCPv6 0 0 312 52 4 : tunables 0 0
0 : slabdata 0 0 0
TCPv6 420 420 2240 14 8 : tunables 0 0
0 : slabdata 30 30 0
cfq_io_cq 1156 1156 120 34 1 : tunables 0 0
0 : slabdata 34 34 0
mqueue_inode_cache 34 34 960 34 8 : tunables 0 0
0 : slabdata 1 1 0
xfs_dqtrx 0 0 528 31 4 : tunables 0 0
0 : slabdata 0 0 0
xfs_rui_item 0 0 672 48 8 : tunables 0 0
0 : slabdata 0 0 0
xfs_rud_item 0 0 152 53 2 : tunables 0 0
0 : slabdata 0 0 0
xfs_ili 144 144 168 48 2 : tunables 0 0
0 : slabdata 3 3 0
xfs_inode 170 170 960 34 8 : tunables 0 0
0 : slabdata 5 5 0
xfs_efd_item 0 0 416 39 4 : tunables 0 0
0 : slabdata 0 0 0
xfs_buf_item 132 132 248 33 2 : tunables 0 0
0 : slabdata 4 4 0
xfs_da_state 0 0 480 34 4 : tunables 0 0
0 : slabdata 0 0 0
xfs_btree_cur 420 420 232 35 2 : tunables 0 0
0 : slabdata 12 12 0
xfs_log_ticket 308 308 184 44 2 : tunables 0 0
0 : slabdata 7 7 0
bio-2 51 51 320 51 4 : tunables 0 0
0 : slabdata 1 1 0
nfs_direct_cache 0 0 336 48 4 : tunables 0 0
0 : slabdata 0 0 0
nfs_read_data 36 36 896 36 8 : tunables 0 0
0 : slabdata 1 1 0
nfs_inode_cache 0 0 1080 30 8 : tunables 0 0
0 : slabdata 0 0 0
isofs_inode_cache 0 0 656 49 8 : tunables 0 0
0 : slabdata 0 0 0
fat_inode_cache 0 0 744 44 8 : tunables 0 0
0 : slabdata 0 0 0
fat_cache 0 0 40 102 1 : tunables 0 0
0 : slabdata 0 0 0
jbd2_journal_head 0 0 120 34 1 : tunables 0 0
0 : slabdata 0 0 0
jbd2_revoke_table_s 0 0 16 256 1 : tunables 0
0 0 : slabdata 0 0 0
ext4_inode_cache 0 0 1088 30 8 : tunables 0 0
0 : slabdata 0 0 0
ext4_allocation_context 0 0 128 32 1 : tunables 0
0 0 : slabdata 0 0 0
ext4_io_end 0 0 64 64 1 : tunables 0 0
0 : slabdata 0 0 0
ext4_extent_status 0 0 40 102 1 : tunables 0 0
0 : slabdata 0 0 0
mbcache 0 0 56 73 1 : tunables 0 0
0 : slabdata 0 0 0
fscrypt_info 0 0 32 128 1 : tunables 0 0
0 : slabdata 0 0 0
fscrypt_ctx 0 0 48 85 1 : tunables 0 0
0 : slabdata 0 0 0
userfaultfd_ctx_cache 0 0 192 42 2 : tunables 0
0 0 : slabdata 0 0 0
dio 0 0 640 51 8 : tunables 0 0
0 : slabdata 0 0 0
pid_namespace 0 0 2232 14 8 : tunables 0 0
0 : slabdata 0 0 0
posix_timers_cache 884 884 240 34 2 : tunables 0 0
0 : slabdata 26 26 0
rpc_inode_cache 0 0 704 46 8 : tunables 0 0
0 : slabdata 0 0 0
ip4-frags 185 185 216 37 2 : tunables 0 0
0 : slabdata 5 5 0
xfrm_dst_cache 0 0 448 36 4 : tunables 0 0
0 : slabdata 0 0 0
RAW 1564 1564 960 34 8 : tunables 0 0
0 : slabdata 46 46 0
tw_sock_TCP 990 990 248 33 2 : tunables 0 0
0 : slabdata 30 30 0
request_sock_TCP 520 520 312 52 4 : tunables 0 0
0 : slabdata 10 10 0
TCP 544 544 2048 16 8 : tunables 0 0
0 : slabdata 34 34 0
hugetlbfs_inode_cache 104 104 624 52 8 : tunables 0
0 0 : slabdata 2 2 0
dquot 0 0 256 32 2 : tunables 0 0
0 : slabdata 0 0 0
request_queue 195 195 2144 15 8 : tunables 0 0
0 : slabdata 13 13 0
blkdev_ioc 1560 1560 104 39 1 : tunables 0 0
0 : slabdata 40 40 0
user_namespace 0 0 440 37 4 : tunables 0 0
0 : slabdata 0 0 0
dmaengine-unmap-256 15 15 2112 15 8 : tunables 0
0 0 : slabdata 1 1 0
dmaengine-unmap-128 1230 1230 1088 30 8 : tunables 0
0 0 : slabdata 41 41 0
sock_inode_cache 6808 6808 704 46 8 : tunables 0 0
0 : slabdata 148 148 0
file_lock_cache 1600 1600 200 40 2 : tunables 0 0
0 : slabdata 40 40 0
fsnotify_mark_connector 5610 5610 24 170 1 : tunables 0
0 0 : slabdata 33 33 0
net_namespace 5 5 5888 5 8 : tunables 0 0
0 : slabdata 1 1 0
taskstats 1813 1813 328 49 4 : tunables 0 0
0 : slabdata 37 37 0
proc_inode_cache 13776 13776 680 48 8 : tunables 0 0
0 : slabdata 287 287 0
sigqueue 2040 2040 160 51 2 : tunables 0 0
0 : slabdata 40 40 0
bdev_cache 909 1287 832 39 8 : tunables 0 0
0 : slabdata 33 33 0
shmem_inode_cache 48414 49864 712 46 8 : tunables 0 0
0 : slabdata 1084 1084 0
kernfs_node_cache 54722 54780 136 30 1 : tunables 0 0
0 : slabdata 1826 1826 0
mnt_cache 1890 1890 384 42 4 : tunables 0 0
0 : slabdata 45 45 0
inode_cache 25175 26447 608 53 8 : tunables 0 0
0 : slabdata 499 499 0
dentry 108066 117096 192 42 2 : tunables 0 0
0 : slabdata 2788 2788 0
iint_cache 0 0 80 51 1 : tunables 0 0
0 : slabdata 0 0 0
buffer_head 585 585 104 39 1 : tunables 0 0
0 : slabdata 15 15 0
vm_area_struct 69560 69760 200 40 2 : tunables 0 0
0 : slabdata 1744 1744 0
mm_struct 4920 4920 1088 30 8 : tunables 0 0
0 : slabdata 164 164 0
files_cache 6624 6624 704 46 8 : tunables 0 0
0 : slabdata 144 144 0
signal_cache 7040 7200 1024 32 8 : tunables 0 0
0 : slabdata 225 225 0
sighand_cache 2670 2670 2112 15 8 : tunables 0 0
0 : slabdata 178 178 0
task_struct 1431 1468 7744 4 8 : tunables 0 0
0 : slabdata 367 367 0
cred_jar 20580 20580 192 42 2 : tunables 0 0
0 : slabdata 490 490 0
Acpi-Operand 21504 21504 72 56 1 : tunables 0 0
0 : slabdata 384 384 0
Acpi-Parse 2920 2920 56 73 1 : tunables 0 0
0 : slabdata 40 40 0
Acpi-State 1836 1836 80 51 1 : tunables 0 0
0 : slabdata 36 36 0
Acpi-Namespace 80882 82416 40 102 1 : tunables 0 0
0 : slabdata 808 808 0
anon_vma_chain 95424 95744 64 64 1 : tunables 0 0
0 : slabdata 1496 1496 0
anon_vma 47886 47886 88 46 1 : tunables 0 0
0 : slabdata 1041 1041 0
pid 7776 7776 128 32 1 : tunables 0 0
0 : slabdata 243 243 0
numa_policy 62 62 264 31 2 : tunables 0 0
0 : slabdata 2 2 0
trace_event_file 2208 2208 88 46 1 : tunables 0 0
0 : slabdata 48 48 0
ftrace_event_field 4675 4675 48 85 1 : tunables 0 0
0 : slabdata 55 55 0
radix_tree_node 1266533 1266720 584 28 4 : tunables 0
0 0 : slabdata 45240 45240 0
task_group 184 184 704 46 8 : tunables 0 0
0 : slabdata 4 4 0
dma-kmalloc-8192 0 0 8192 4 8 : tunables 0 0
0 : slabdata 0 0 0
dma-kmalloc-4096 0 0 4096 8 8 : tunables 0 0
0 : slabdata 0 0 0
dma-kmalloc-2048 0 0 2048 16 8 : tunables 0 0
0 : slabdata 0 0 0
dma-kmalloc-1024 0 0 1024 32 8 : tunables 0 0
0 : slabdata 0 0 0
dma-kmalloc-512 0 0 512 32 4 : tunables 0 0
0 : slabdata 0 0 0
dma-kmalloc-256 0 0 256 32 2 : tunables 0 0
0 : slabdata 0 0 0
dma-kmalloc-128 0 0 128 32 1 : tunables 0 0
0 : slabdata 0 0 0
dma-kmalloc-64 0 0 64 64 1 : tunables 0 0
0 : slabdata 0 0 0
dma-kmalloc-32 0 0 32 128 1 : tunables 0 0
0 : slabdata 0 0 0
dma-kmalloc-16 0 0 16 256 1 : tunables 0 0
0 : slabdata 0 0 0
dma-kmalloc-8 0 0 8 512 1 : tunables 0 0
0 : slabdata 0 0 0
dma-kmalloc-192 0 0 192 42 2 : tunables 0 0
0 : slabdata 0 0 0
dma-kmalloc-96 0 0 96 42 1 : tunables 0 0
0 : slabdata 0 0 0
kmalloc-8192 1292 1292 8192 4 8 : tunables 0 0
0 : slabdata 323 323 0
kmalloc-4096 1056 1056 4096 8 8 : tunables 0 0
0 : slabdata 132 132 0
kmalloc-2048 3246 3312 2048 16 8 : tunables 0 0
0 : slabdata 207 207 0
kmalloc-1024 9056 9056 1024 32 8 : tunables 0 0
0 : slabdata 283 283 0
kmalloc-512 7487 7488 512 32 4 : tunables 0 0
0 : slabdata 234 234 0
kmalloc-256 17544 18528 256 32 2 : tunables 0 0
0 : slabdata 579 579 0
kmalloc-192 10416 10416 192 42 2 : tunables 0 0
0 : slabdata 248 248 0
kmalloc-128 6756 6816 128 32 1 : tunables 0 0
0 : slabdata 213 213 0
kmalloc-96 48655 49308 96 42 1 : tunables 0 0
0 : slabdata 1174 1174 0
kmalloc-64 23650 23808 64 64 1 : tunables 0 0
0 : slabdata 372 372 0
kmalloc-32 34432 34432 32 128 1 : tunables 0 0
0 : slabdata 269 269 0
kmalloc-16 27392 27392 16 256 1 : tunables 0 0
0 : slabdata 107 107 0
kmalloc-8 37376 37376 8 512 1 : tunables 0 0
0 : slabdata 73 73 0
kmem_cache_node 1725 1728 64 64 1 : tunables 0 0
0 : slabdata 27 27 0
kmem_cache 966 966 384 42 4 : tunables 0 0
0 : slabdata 23 23 0
To illustrate the issue I'm issuing the simplest command I can come up with:
$ date; sudo touch /mnt/empty
Thu Nov 29 20:09:35 UTC 2018
Now let's see what happens in 10s surrounding this event. Please read
through all events to see the whole picture.
1. Memory reclaim is happening as it should (my working set is bigger than RAM):
$ sudo /usr/share/bcc/tools/funclatency -mTi 1 shrink_node
Tracing 1 functions for "shrink_node"... Hit Ctrl-C to end.
...
20:09:30
msecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 2 |************************** |
8 -> 15 : 1 |************* |
16 -> 31 : 3 |****************************************|
20:09:31
msecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 2 |****************************************|
8 -> 15 : 1 |******************** |
20:09:32
msecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 3 |****************************************|
20:09:33
msecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 2 |************************** |
8 -> 15 : 1 |************* |
16 -> 31 : 3 |****************************************|
20:09:34
20:09:35
msecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 2 |****************************************|
8 -> 15 : 2 |****************************************|
16 -> 31 : 2 |****************************************|
20:09:36
msecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 1 |******************** |
8 -> 15 : 1 |******************** |
16 -> 31 : 2 |****************************************|
32 -> 63 : 2 |****************************************|
20:09:37
20:09:38
msecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 2 |****************************************|
8 -> 15 : 2 |****************************************|
16 -> 31 : 2 |****************************************|
20:09:39
20:09:40
msecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 2 |****************************************|
8 -> 15 : 2 |****************************************|
16 -> 31 : 2 |****************************************|
2. XFS reclaim only kicks in when I create an empty file, silence otherwise:
$ sudo /usr/share/bcc/tools/funclatency -mTi 1 xfs_fs_free_cached_objects
Tracing 1 functions for "xfs_fs_free_cached_objects"... Hit Ctrl-C to end.
...
20:09:30
20:09:31
20:09:32
20:09:33
20:09:34
20:09:35
20:09:36
20:09:37
msecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 0 | |
32 -> 63 : 2 |****************************************|
20:09:38
20:09:39
20:09:40
3. We can see that XFS inode is actually reclaimed from kswapd0:
$ sudo /usr/share/bcc/tools/stackcount -Ti 1 xfs_reclaim_inode
Tracing 1 functions for "xfs_reclaim_inode"... Hit Ctrl-C to end.
...
20:09:30
20:09:31
20:09:32
20:09:33
20:09:34
20:09:35
20:09:36
xfs_reclaim_inode
xfs_reclaim_inodes_ag
xfs_reclaim_inodes_nr
kretprobe_trampoline
kswapd0 [1862]
1
20:09:37
20:09:38
20:09:39
20:09:40
4. And we can also see that flush on disk is happening from the same kswapd0:
$ sudo /usr/share/bcc/tools/stackcount -Ti 1 xfs_iflush
Tracing 1 functions for "xfs_iflush"... Hit Ctrl-C to end.
...
20:09:30
20:09:31
20:09:32
20:09:33
20:09:34
20:09:35
20:09:36
20:09:37
xfs_iflush
xfs_reclaim_inode
xfs_reclaim_inodes_ag
xfs_reclaim_inodes_nr
kretprobe_trampoline
kswapd0 [1862]
1
20:09:38
20:09:39
20:09:40
5. Back to XFS reclaim source locations, both kswapd0 and kswapd1 hit it:
ivan@36s336:~$ sudo /usr/share/bcc/tools/stackcount -Ti 1
xfs_fs_free_cached_objects
Tracing 1 functions for "xfs_fs_free_cached_objects"... Hit Ctrl-C to end.
...
20:09:30
20:09:31
20:09:32
20:09:33
20:09:34
20:09:35
20:09:36
20:09:37
xfs_fs_free_cached_objects
super_cache_scan
shrink_slab.part.45
shrink_node
kretprobe_trampoline
kswapd1 [1863]
1
xfs_fs_free_cached_objects
super_cache_scan
shrink_slab.part.45
shrink_node
kretprobe_trampoline
kswapd0 [1862]
1
20:09:38
20:09:39
20:09:40
6. And we can see that it was kswapd0 that flushed data to disk synchronously:
$ sudo /usr/share/bcc/tools/stackcount -Ti 1 xfs_buf_submit_wait
Tracing 1 functions for "xfs_buf_submit_wait"... Hit Ctrl-C to end.
...
20:09:30
20:09:31
20:09:32
20:09:33
20:09:34
20:09:35
20:09:36
xfs_buf_submit_wait
xfs_bwrite
xfs_reclaim_inode
xfs_reclaim_inodes_ag
xfs_reclaim_inodes_nr
kretprobe_trampoline
kswapd0 [1862]
1
20:09:37
20:09:38
20:09:39
20:09:40
7. Finally, we only see xfs_fs_nr_cached_objects return non-zero
around the same time:
$ sudo /usr/share/bcc/tools/trace -T 'r::xfs_fs_nr_cached_objects
(retval > 0) "cached = %d", retval'
TIME PID TID COMM FUNC -
20:09:36 1862 1862 kswapd0 xfs_fs_nr_cached_objects cached = 1
20:09:36 1862 1862 kswapd0 xfs_fs_nr_cached_objects cached = 1
20:09:36 1863 1863 kswapd1 xfs_fs_nr_cached_objects cached = 1
20:09:36 1863 1863 kswapd1 xfs_fs_nr_cached_objects cached = 1
You can see that have 249GB of page cache:
$ free -h
total used free shared buff/cache available
Mem: 251G 1.0G 1.1G 656M 249G 248G
Swap: 0B 0B 0B
And the number of dirty pages is zero:
$ cat /proc/vmstat
nr_free_pages 295648
nr_zone_inactive_anon 99463
nr_zone_active_anon 76344
nr_zone_inactive_file 56796555
nr_zone_active_file 8297372
nr_zone_unevictable 0
nr_zone_write_pending 0
nr_mlock 0
nr_page_table_pages 1578
nr_kernel_stack 9168
nr_bounce 0
nr_free_cma 0
numa_hit 99220256
numa_miss 52104657
numa_foreign 52104657
numa_interleave 135792
numa_local 99127058
numa_other 52197855
nr_inactive_anon 99463
nr_active_anon 76344
nr_inactive_file 56796555
nr_active_file 8297372
nr_unevictable 0
nr_slab_reclaimable 194565
nr_slab_unreclaimable 43128
nr_isolated_anon 0
nr_isolated_file 0
workingset_refault 69892763
workingset_activate 289419
workingset_nodereclaim 0
nr_anon_pages 7700
nr_mapped 5765
nr_file_pages 65262134
nr_dirty 0
nr_writeback 0
nr_writeback_temp 0
nr_shmem 168119
nr_shmem_hugepages 0
nr_shmem_pmdmapped 0
nr_anon_transparent_hugepages 0
nr_unstable 0
nr_vmscan_write 0
nr_vmscan_immediate_reclaim 0
nr_dirtied 0
nr_written 0
nr_dirty_threshold 13017651
nr_dirty_background_threshold 6500878
pgpgin 594234184
pgpgout 2360
pswpin 0
pswpout 0
pgalloc_dma 0
pgalloc_dma32 348991
pgalloc_normal 151243219
pgalloc_movable 0
allocstall_dma 0
allocstall_dma32 0
allocstall_normal 0
allocstall_movable 0
pgskip_dma 0
pgskip_dma32 0
pgskip_normal 0
pgskip_movable 0
pgfree 151892594
pgactivate 8963375
pgdeactivate 742721
pglazyfree 0
pgfault 3796959
pgmajfault 0
pglazyfreed 0
pgrefill 742721
pgsteal_kswapd 83442196
pgsteal_direct 0
pgscan_kswapd 83442409
pgscan_direct 0
pgscan_direct_throttle 0
zone_reclaim_failed 0
pginodesteal 0
slabs_scanned 26710
kswapd_inodesteal 0
kswapd_low_wmark_hit_quickly 0
kswapd_high_wmark_hit_quickly 0
pageoutrun 1636
pgrotated 0
drop_pagecache 0
drop_slab 0
oom_kill 0
numa_pte_updates 0
numa_huge_pte_updates 0
numa_hint_faults 0
numa_hint_faults_local 0
numa_pages_migrated 0
pgmigrate_success 0
pgmigrate_fail 0
compact_migrate_scanned 0
compact_free_scanned 0
compact_isolated 0
compact_stall 0
compact_fail 0
compact_success 0
compact_daemon_wake 0
compact_daemon_migrate_scanned 0
compact_daemon_free_scanned 0
htlb_buddy_alloc_success 0
htlb_buddy_alloc_fail 0
unevictable_pgs_culled 18354
unevictable_pgs_scanned 0
unevictable_pgs_rescued 19851
unevictable_pgs_mlocked 19851
unevictable_pgs_munlocked 19851
unevictable_pgs_cleared 0
unevictable_pgs_stranded 0
thp_fault_alloc 0
thp_fault_fallback 0
thp_collapse_alloc 0
thp_collapse_alloc_failed 0
thp_file_alloc 0
thp_file_mapped 0
thp_split_page 0
thp_split_page_failed 0
thp_deferred_split_page 0
thp_split_pmd 0
thp_split_pud 0
thp_zero_page_alloc 0
thp_zero_page_alloc_failed 0
thp_swpout 0
thp_swpout_fallback 0
balloon_inflate 0
balloon_deflate 0
balloon_migrate 0
swap_ra 0
swap_ra_hit 0
It's zero all the time, except for when I touch an empty file. There's
just no other write IO on the system.
The whole issue here is that synchronous IO from memory reclaim is
unbound in time. For all I know we can have 10TB drive that runs at
floppy drive speeds or worse.
The following can easily happen (correct me if it can't for some reason):
1. kswapd gets stuck because of slow storage and memory is not getting reclaimed
2. memory allocation doesn't have any free pages and kicks in direct reclaim
3. direct reclaim is stuck behind kswapd
I'm not sure why you say direct reclaim happens first, allocstall is zero.
> > The catalyst of our issue is terrible disks. It's not uncommon to see
> > the following stack in hung task detector:
> >
> > Nov 15 21:55:13 21m21 kernel: INFO: task some-task:156314 blocked for
> > more than 10 seconds.
> > Nov 15 21:55:13 21m21 kernel: Tainted: G O
> > 4.14.59-cloudflare-2018.7.5 #1
> > Nov 15 21:55:13 21m21 kernel: "echo 0 >
> > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > Nov 15 21:55:13 21m21 kernel: some-task D11792 156314 156183 0x00000080
> > Nov 15 21:55:13 21m21 kernel: Call Trace:
> > Nov 15 21:55:13 21m21 kernel: ? __schedule+0x21a/0x820
> > Nov 15 21:55:13 21m21 kernel: schedule+0x28/0x80
> > Nov 15 21:55:13 21m21 kernel: schedule_preempt_disabled+0xa/0x10
> > Nov 15 21:55:13 21m21 kernel: __mutex_lock.isra.2+0x16a/0x490
> > Nov 15 21:55:13 21m21 kernel: ? xfs_reclaim_inodes_ag+0x265/0x2d0
> > Nov 15 21:55:13 21m21 kernel: xfs_reclaim_inodes_ag+0x265/0x2d0
> > Nov 15 21:55:13 21m21 kernel: ? kmem_cache_alloc+0x14d/0x1b0
> > Nov 15 21:55:13 21m21 kernel: ? radix_tree_gang_lookup_tag+0xc4/0x130
> > Nov 15 21:55:13 21m21 kernel: ? __list_lru_walk_one.isra.5+0x33/0x130
> > Nov 15 21:55:13 21m21 kernel: xfs_reclaim_inodes_nr+0x31/0x40
> > Nov 15 21:55:13 21m21 kernel: super_cache_scan+0x156/0x1a0
> > Nov 15 21:55:13 21m21 kernel: shrink_slab.part.51+0x1d2/0x3a0
> > Nov 15 21:55:13 21m21 kernel: shrink_node+0x113/0x2e0
> > Nov 15 21:55:13 21m21 kernel: do_try_to_free_pages+0xb3/0x310
> > Nov 15 21:55:13 21m21 kernel: try_to_free_pages+0xd2/0x190
> > Nov 15 21:55:13 21m21 kernel: __alloc_pages_slowpath+0x3a3/0xdc0
> > Nov 15 21:55:13 21m21 kernel: ? ip_output+0x5c/0xc0
> > Nov 15 21:55:13 21m21 kernel: ? update_curr+0x141/0x1a0
> > Nov 15 21:55:13 21m21 kernel: __alloc_pages_nodemask+0x223/0x240
> > Nov 15 21:55:13 21m21 kernel: skb_page_frag_refill+0x93/0xb0
> > Nov 15 21:55:13 21m21 kernel: sk_page_frag_refill+0x19/0x80
> > Nov 15 21:55:13 21m21 kernel: tcp_sendmsg_locked+0x247/0xdc0
> > Nov 15 21:55:13 21m21 kernel: tcp_sendmsg+0x27/0x40
> > Nov 15 21:55:13 21m21 kernel: sock_sendmsg+0x36/0x40
> > Nov 15 21:55:13 21m21 kernel: sock_write_iter+0x84/0xd0
> > Nov 15 21:55:13 21m21 kernel: __vfs_write+0xdd/0x140
> > Nov 15 21:55:13 21m21 kernel: vfs_write+0xad/0x1a0
> > Nov 15 21:55:13 21m21 kernel: SyS_write+0x42/0x90
> > Nov 15 21:55:13 21m21 kernel: do_syscall_64+0x60/0x110
> > Nov 15 21:55:13 21m21 kernel: entry_SYSCALL_64_after_hwframe+0x3d/0xa2
> >
> > Here "some-task" is trying to send some bytes over network and it's
> > stuck in direct reclaim. Naturally, kswapd is not keeping up with its
> > duties.
>
> That's not kswapd causing the problem here, that's direct reclaim.
I think direct reclaim happens concurrently with kswapd, see my
workload example.
> > It seems to me that our terrible disks sometimes take a pause to think
> > about the meaning of life for a few seconds.
>
> You've probably got lots of dirty inodes backed up waiting for IO.
I don't have lots of dirty inodes waiting on IO.
> How big are xfs inode slab caches (slabinfo output, please)? How big are the logs
> on the filesystems on the machine (xfs_info output, please)?
See my workload example.
> > During that time XFS
> > shrinker is stuck, which drives the whole system out of free memory
> > and in turns triggers direct reclaim.
>
> It happens the other way around. What you have here is direct
> reclaim trying to get the reclaim lock for a given AG to be able to
> reclaim inodes from it. direct reclaim only gets stuck here when all
> AGs are already being reclaimed from (i.e. already under direct
> reclaim).
>
> > One solution to this is to not go into direct reclaim by keeping more
> > free pages with vm.watermark_scale_factor, but I'd like to discard
> > this and argue that we're going to hit direct reclaim at some point
> > anyway.
>
> Right, but the problem is that the mm/ subsystem allows effectively
> unbound direct reclaim concurrency. At some point, having tens to
> hundreds of direct reclaimers all trying to write dirty inodes to
> disk causes catastrophic IO breakdown and everything grinds to a
> halt forever. We have to prevent that breakdown from occurring.
>
> i.e. we have to throttle direct reclaim to before it reaches IO
> breakdown /somewhere/. The memory reclaim subsystem does not do it,
> so we have to do it in XFS itself. The problem here is that if we
> ignore direct reclaim (i.e do nothing rather than block waiting on
> reclaim progress) then the mm/ reclaim algorithms will eventually
> think they aren't making progress and unleash the OOM killer.
>
> > The solution I have in mind for this is not to try to write anything
> > to (disastrously terrible) storage in shrinkers. We have 95GB of page
> > cache readily available for reclaim and it seems a lot cheaper to grab
> > that.
>
> The mm/ subsystem tries to keep a balance between page cache and
> shrinker controlled caches. /proc/sys/vm/vfs_cache_pressure
> controls the balance between page cache and inode/dentry cache
> reclaim pressure..
>
> > That brings me to the first question around memory subsystem: are
> > shrinkers supposed to flush any dirty data?
>
> No shrinker does that, but if they wanted to they would be perfectly
> within their right to write back data.
>
> XFS is writing back /metadata/ here because direct reclaim is
> putting excessive pressure on the inode cache and not allowing the
> background cleaners to do their job.
Metadata is still data, XFS still writes it to the same block device.
> > My gut feeling is that
> > they should not do that, because there's already writeback mechanism
> > with own tunables for limits to take care of that. If a system runs
> > out of memory reclaimable without IO and dirty pages are under limit,
> > it's totally fair to OOM somebody.
> >
> > It's totally possible that I'm wrong about this feeling, but either
> > way I think docs need an update on this matter:
> >
> > * https://elixir.bootlin.com/linux/v4.14.55/source/Documentation/filesystems/vfs.txt
> >
> > nr_cached_objects: called by the sb cache shrinking function for the
> > filesystem to return the number of freeable cached objects it contains.
>
> You are assuming that "freeable" means "instantly freeable object",
> not "unreferenced object that can be freed in the near future". We
> use the latter definition in the shrinkers, not the former.
I'm only assuming things because documentation leaves room for
interpretation. I would love to this worded in a way that's crystal
clear and mentions possibility of IO.
> > My second question is conditional on the first one: if filesystems are
> > supposed to flush dirty data in response to shrinkers, then how can I
> > stop this, given my knowledge about combination of lots of available
> > page cache and terrible disks?
>
> Filesystems have more caches that just the page cache.
>
> > I've tried two things to address this problem ad-hoc.
> >
> > 1. Run the following systemtap script to trick shrinkers into thinking
> > that XFS has nothing to free:
> >
> > probe kernel.function("xfs_fs_nr_cached_objects").return {
> > $return = 0
> > }
> >
> > That did the job and shrink_node latency dropped considerably, while
> > calls to xfs_fs_free_cached_objects disappeared.
>
> Which effectively turned off direct reclaim for XFS inodes. See
> above - this just means that when you have no easily reclaimable
> page cache the system will OOM kill rather than wait for inodes to
> be reclaimed. i.e. it looks good until everything suddenly goes
> wrong and then everything dies a horrible death.
We have hundreds of gigabytes of page cache, dirty pages are not
allowed to go near that mark. There's a separate limit for dirty data.
What I want to have is a way to tell the kernel to not try to flush
data to disk in response to reclaim, because that's choosing a very
real horrible life over imaginary horrible death. I can't possibly
create enough dirty inodes to cause the horrible death you describe.
This may be the case for low memory machines, but not for high memory
(which are arguably a commodity these days).
Can we have a sysctl toggle for no IO during reclaim?
> > 2. Use vm.vfs_cache_pressure to do the same thing. This failed
> > miserably, because of the following code in super_cache_count:
> >
> > if (sb->s_op && sb->s_op->nr_cached_objects)
> > total_objects = sb->s_op->nr_cached_objects(sb, sc);
> >
> > total_objects += list_lru_shrink_count(&sb->s_dentry_lru, sc);
> > total_objects += list_lru_shrink_count(&sb->s_inode_lru, sc);
> >
> > total_objects = vfs_pressure_ratio(total_objects);
> > return total_objects;
> >
> > XFS was doing its job cleaning up inodes with the background mechanims
> > it has (m_reclaim_workqueue), but kernel also stopped cleaning up
> > readily available inodes after XFS.
>
> You mean the kernel stopped handing readily available inodes to XFS
> to free.
>
> ie. The VFS has to release unreferenced inodes before they are
> handed to XFS to clean up and free. IOWs, I suspect you biased
> vm.vfs_cache_pressure to turn off inode reclaim to effectively do
> the same thing as above. However, by stopping the feed of
> unreferenced inodes to XFS you stopped inode reclaim altogether and
> that caused other problems.
>
> What I suspect you want to do is bias vfs_cache_pressure the other
> way, so that it /agressively/ feeds inode reclaim and so prevent
> large numbers of inodes from building up in the cache. That way
> reclaim won't have as many dirty inodes to block on when a sustained
> memory shortage drives large amounts of direct reclaim into the
> shrinkers...
No, I absolutely do not want to do that, because it still means writes
to disk in response to reclaim. That's the opposite of what I want.
> > I'm not a kernel hacker and to be honest with you I don't even
> > understand all the nuances here. All I know is:
> >
> > 1. I have lots of page cache and terrible disks.
> > 2. I want to reclaim page cache and never touch disks in response to
> > memory reclaim.
> > 3. Direct reclaim will happen at some point, somebody will want a big
> > chunk of memory all at once.
> > 4. I'm probably ok with reclaiming clean xfs inodes synchronously in
> > reclaim path.
> >
> > This brings me to my final question: what should I do to avoid latency
> > in reclaim (direct or kswapd)?
> >
> > To reiterate the importance of this issue: we see interactive
> > applications with zero IO stall for multiple seconds in writes to
> > non-blocking sockets and page faults on newly allocated memory, while
> > 95GB of memory is in page cache.
>
> It really just sounds like you are allowing too many dirty inodes to
> build up in memory. You have a huge amount of memory but really slow
> disks - that's just asking for idata and metadata writeback latency
> problems.
I don't care as much about idata and metadata writeback latency.
Writes are asynchronous as long as dirty page limit is not breached
and I don't breach it. What I care about is system freezes when
there's tons of memory to reclaim from page cache, because reclaim
writes to storage synchronously.
> It sounds like, you need to cause inode cleaning to happen
> earlier/faster than it is happening now, because your disks are slow
> and so you can't allow a big backlog to build up. Reducing
> /proc/sys/fs/xfs/xfssyncd_centisecs can get background metadata
> writeback started earlier. As per Documentation/filesystems/xfs.txt:
>
> fs.xfs.xfssyncd_centisecs (Min: 100 Default: 3000 Max: 720000)
> The interval at which the filesystem flushes metadata
> out to disk and runs internal cache cleanup routines.
>
> If this doesn't help, then I'd suggest taht your config is simply
> allowing too many dirty inodes to build up in memory. With XFS, the
> number of dirty inodes that can be held in memory at any point in
> time is bound by the size of the XFS log in the filesystem. Hence if
> you have hundreds of MB of journal space, then you can cache
> hundreds of thousands of dirty inodes in memory and so can take tens
> of minutes to write them back to slow disks. So if earlier writeback
> doesn't help, I'd suggest that using smaller logs with your
> filesystems is the only real option here....
You are right that our config allows too many dirty inodes to build up
in memory. Unfortunately, too many means any number larger than zero
and lowering fs.xfs.xfssyncd_centisecs to 600 made no difference in
real production workload.
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
