RE: [RFC PATCH] dcache: give a chance to yield in shrink_dentry_list

[Biscuit Ninja]

> A softlockup in shrink_dentry_list when called from shrink_dcache_sb
> was observed on a very busy server. It's possible that the list
> passed to shrink_dentry_list is so big that it takes a while to
> dispose of all entries. Adding a simple cond_resched would give
> the cpu a chance to do some other useful work and no trip the
> softlockup watchdog.

Hello Nikolay,

I'm interested in your proposed patch because I believe I've come across
the same issue a handful of times, typically:

WARNING: CPU: 0 PID: 22717
at /build/buildd/linux-lts-trusty-3.13.0/fs/dcache.c:362 d_shrink_del
+0x79/0x90()
Modules linked in: pcnet32 mii vmwgfx ttm drm psmouse lp i2c_piix4 ppdev
parport_pc vmw_vmci shpchp serio_raw mac_hid vmw_balloon parport floppy
mptspi mptscsih mptbase e1000 vmw_pvscsi vmxnet3
CPU: 0 PID: 22717 Comm: sh Not tainted 3.13.0-32-generic
#57~precise1-Ubuntu
Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop
Reference Platform, BIOS 6.00 10/22/2013
 000000000000016a ffff88000616bbe8 ffffffff81752c9e 0000000000000007
 0000000000000000 ffff88000616bc28 ffffffff8106af7c ffff88000616bc08
 ffff880037f54480 ffff880037f54480 ffff880037f54500 ffff880037f544d8
Call Trace:
 [<ffffffff81752c9e>] dump_stack+0x46/0x58
 [<ffffffff8106af7c>] warn_slowpath_common+0x8c/0xc0
 [<ffffffff8106afca>] warn_slowpath_null+0x1a/0x20
 [<ffffffff811df419>] d_shrink_del+0x79/0x90
 [<ffffffff811e0ad2>] shrink_dentry_list+0x62/0xe0
 [<ffffffff811e0c18>] shrink_dcache_parent+0x28/0x70
 [<ffffffff812321c7>] proc_flush_task_mnt.isra.15+0x77/0x170
 [<ffffffff812356e6>] proc_flush_task+0x56/0x70
 [<ffffffff8106c803>] release_task+0x33/0x130
 [<ffffffff8106cdcf>] wait_task_zombie+0x4cf/0x5f0
 [<ffffffff8106cffb>] wait_consider_task.part.8+0x10b/0x180
 [<ffffffff8106d0d5>] wait_consider_task+0x65/0x70
 [<ffffffff8106d1e1>] do_wait+0x101/0x260
 [<ffffffff8106e213>] SyS_wait4+0xa3/0x100
 [<ffffffff8106bc10>] ? task_stopped_code+0x50/0x50
 [<ffffffff8176847f>] tracesys+0xe1/0xe6
---[ end trace e2a4a7d8b698f0ea ]---
BUG: soft lockup - CPU#0 stuck for 22s! [sh:22717]
Modules linked in: pcnet32 mii vmwgfx ttm drm psmouse lp i2c_piix4 ppdev
parport_pc vmw_vmci shpchp serio_raw mac_hid vmw_balloon parport floppy
mptspi mptscsih mptbase e1000 vmw_pvscsi vmxnet3
CPU: 0 PID: 22717 Comm: sh Tainted: G        W    3.13.0-32-generic
#57~precise1-Ubuntu
Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop
Reference Platform, BIOS 6.00 10/22/2013
task: ffff88001ef717f0 ti: ffff88000616a000 task.ti: ffff88000616a000
RIP: 0010:[<ffffffff8175f28c>]  [<ffffffff8175f28c>] _raw_spin_lock
+0x1c/0x50
RSP: 0018:ffff88000616bc58  EFLAGS: 00000246


This is under Ubuntu 12.04.5 with kernel 3.13.0-32-generic.