[RFC PATCH] cgroup, netclassid: add a preemption point to write_classid

[Michal Hocko]

From: Michal Hocko <mhocko@xxxxxxxx>

We have seen a customer complaining about soft lockups on !PREEMPT
kernel config with 4.4 based kernel

[1072141.435366] NMI watchdog: BUG: soft lockup - CPU#21 stuck for 22s! [systemd:1]
[1072141.444090] Modules linked in: mpt3sas raid_class binfmt_misc af_packet 8021q garp mrp stp llc xfs libcrc32c bonding iscsi_ibft iscsi_boot_sysfs msr ext4 crc16 jbd2 mbcache cdc_ether usbnet mii joydev hid_generic usbhid intel_rapl x86_pkg_temp_thermal intel_powerclamp coretemp crct10dif_pclmul crc32_pclmul ghash_clmulni_intel ipmi_ssif mgag200 i2c_algo_bit ttm ipmi_devintf drbg ixgbe drm_kms_helper vxlan ansi_cprng ip6_udp_tunnel drm aesni_intel udp_tunnel aes_x86_64 iTCO_wdt syscopyarea ptp xhci_pci lrw iTCO_vendor_support pps_core gf128mul ehci_pci glue_helper sysfillrect mdio pcspkr sb_edac ablk_helper cryptd ehci_hcd sysimgblt xhci_hcd fb_sys_fops edac_core mei_me lpc_ich ses usbcore enclosure dca mfd_core ipmi_si mei i2c_i801 scsi_transport_sas usb_common ipmi_msghandler shpchp fjes wmi processor button acpi_pad btrfs xor raid6_pq sd_mod crc32c_intel megaraid_sas sg dm_multipath dm_mod scsi_dh_rdac scsi_dh_emc scsi_dh_alua scsi_mod md_mod autofs4
[1072141.444146] Supported: Yes
[1072141.444149] CPU: 21 PID: 1 Comm: systemd Not tainted 4.4.121-92.80-default #1
[1072141.444150] Hardware name: LENOVO Lenovo System x3650 M5 -[5462P4U]- -[5462P4U]-/01GR451, BIOS -[TCE136H-2.70]- 06/13/2018
[1072141.444151] task: ffff880191bd0040 ti: ffff880191bd4000 task.ti: ffff880191bd4000
[1072141.444153] RIP: 0010:[<ffffffff815229f9>]  [<ffffffff815229f9>] update_classid_sock+0x29/0x40
[1072141.444157] RSP: 0018:ffff880191bd7d58  EFLAGS: 00000286
[1072141.444158] RAX: ffff883b177cb7c0 RBX: 0000000000000000 RCX: 0000000000000000
[1072141.444159] RDX: 00000000000009c7 RSI: ffff880191bd7d5c RDI: ffff8822e29bb200
[1072141.444160] RBP: ffff883a72230980 R08: 0000000000000101 R09: 0000000000000000
[1072141.444161] R10: 0000000000000008 R11: f000000000000000 R12: ffffffff815229d0
[1072141.444162] R13: 0000000000000000 R14: ffff881fd0a47ac0 R15: ffff880191bd7f28
[1072141.444163] FS:  00007f3e2f1eb8c0(0000) GS:ffff882000340000(0000) knlGS:0000000000000000
[1072141.444164] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[1072141.444165] CR2: 00007f3e2f200000 CR3: 0000001ffea4e000 CR4: 00000000001606f0
[1072141.444166] Stack:
[1072141.444166]  ffffffa800000246 00000000000009c7 ffffffff8121d583 ffff8818312a05c0
[1072141.444168]  ffff8818312a1100 ffff880197c3b280 ffff881861422858 ffffffffffffffea
[1072141.444170]  ffffffff81522b1c ffffffff81d0ca20 ffff8817fa17b950 ffff883fdd8121e0
[1072141.444171] Call Trace:
[1072141.444179]  [<ffffffff8121d583>] iterate_fd+0x53/0x80
[1072141.444182]  [<ffffffff81522b1c>] write_classid+0x4c/0x80
[1072141.444187]  [<ffffffff8111328b>] cgroup_file_write+0x9b/0x100
[1072141.444193]  [<ffffffff81278bcb>] kernfs_fop_write+0x11b/0x150
[1072141.444198]  [<ffffffff81201566>] __vfs_write+0x26/0x100
[1072141.444201]  [<ffffffff81201bed>] vfs_write+0x9d/0x190
[1072141.444203]  [<ffffffff812028c2>] SyS_write+0x42/0xa0
[1072141.444207]  [<ffffffff815f58c3>] entry_SYSCALL_64_fastpath+0x1e/0xca
[1072141.445490] DWARF2 unwinder stuck at entry_SYSCALL_64_fastpath+0x1e/0xca

If a cgroup has many tasks with many open file descriptors then we would
end up in a large loop without any rescheduling point throught the
operation. Add cond_resched once per task.

Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
Hi,
I am sending this as an RFC because I am not sure this is the best
approach to be honest. The customer who reported this issue was not
entirely thrilled about testing this so it might be possible that the
patch is not sufficient. Details about the workload are not very
detailed either.

Any comment would be highly appreciated.

 net/core/netclassid_cgroup.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/core/netclassid_cgroup.c b/net/core/netclassid_cgroup.c
index 5e4f04004a49..7bf833598615 100644
--- a/net/core/netclassid_cgroup.c
+++ b/net/core/netclassid_cgroup.c
@@ -106,6 +106,7 @@ static int write_classid(struct cgroup_subsys_state *css, struct cftype *cft,
 		iterate_fd(p->files, 0, update_classid_sock,
 			   (void *)(unsigned long)cs->classid);
 		task_unlock(p);
+		cond_resched();
 	}
 	css_task_iter_end(&it);
 
-- 
2.19.0