[PATCH] btrfs: Relinquish CPUs in btrfs_compare_trees

[PATCH] btrfs: Relinquish CPUs in btrfs_compare_trees

[Nikolay Borisov]

When doing any form of incremental send the parent and the child trees
need to be compared via btrfs_compare_trees. This  can result in long
loop chains without ever relinquishing the CPU. This causes softlockup
detector to trigger when comparing trees with a lot of items. Example
report:

watchdog: BUG: soft lockup - CPU#0 stuck for 24s! [snapperd:16153]
CPU: 0 PID: 16153 Comm: snapperd Not tainted 5.2.9-1-default #1 openSUSE Tumbleweed (unreleased)
Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
pstate: 40000005 (nZcv daif -PAN -UAO)
pc : __ll_sc_arch_atomic_sub_return+0x14/0x20
lr : btrfs_release_extent_buffer_pages+0xe0/0x1e8 [btrfs]
sp : ffff00001273b7e0
Call trace:
 __ll_sc_arch_atomic_sub_return+0x14/0x20
 release_extent_buffer+0xdc/0x120 [btrfs]
 free_extent_buffer.part.0+0xb0/0x118 [btrfs]
 free_extent_buffer+0x24/0x30 [btrfs]
 btrfs_release_path+0x4c/0xa0 [btrfs]
 btrfs_free_path.part.0+0x20/0x40 [btrfs]
 btrfs_free_path+0x24/0x30 [btrfs]
 get_inode_info+0xa8/0xf8 [btrfs]
 finish_inode_if_needed+0xe0/0x6d8 [btrfs]
 changed_cb+0x9c/0x410 [btrfs]
 btrfs_compare_trees+0x284/0x648 [btrfs]
 send_subvol+0x33c/0x520 [btrfs]
 btrfs_ioctl_send+0x8a0/0xaf0 [btrfs]
 btrfs_ioctl+0x199c/0x2288 [btrfs]
 do_vfs_ioctl+0x4b0/0x820
 ksys_ioctl+0x84/0xb8
 __arm64_sys_ioctl+0x28/0x38
 el0_svc_common.constprop.0+0x7c/0x188
 el0_svc_handler+0x34/0x90
 el0_svc+0x8/0xc

Fix this by adding a call to cond_resched at the beginning of the main
loop in btrfs_compare_trees.

Fixes: 7069830a9e38 ("Btrfs: add btrfs_compare_trees function")
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/send.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index f856d6ca3771..f3215028235c 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -6757,6 +6757,7 @@ static int btrfs_compare_trees(struct btrfs_root *left_root,
 	advance_left = advance_right = 0;
 
 	while (1) {
+		cond_resched();
 		if (advance_left && !left_end_reached) {
 			ret = tree_advance(left_path, &left_level,
 					left_root_level,
-- 
2.17.1

Re: [PATCH] btrfs: Relinquish CPUs in btrfs_compare_trees

[Johannes Thumshirn]

Looks good,
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>

-- 
Johannes Thumshirn                            SUSE Labs Filesystems
jthumshirn@suse.de                                +49 911 74053 689
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5
90409 Nürnberg
Germany
(HRB 247165, AG München)
Key fingerprint = EC38 9CAB C2C4 F25D 8600 D0D0 0393 969D 2D76 0850

Re: [PATCH] btrfs: Relinquish CPUs in btrfs_compare_trees

[David Sterba]

On Wed, Sep 04, 2019 at 07:33:58PM +0300, Nikolay Borisov wrote:
> When doing any form of incremental send the parent and the child trees
> need to be compared via btrfs_compare_trees. This  can result in long
> loop chains without ever relinquishing the CPU. This causes softlockup
> detector to trigger when comparing trees with a lot of items. Example
> report:
> 
> watchdog: BUG: soft lockup - CPU#0 stuck for 24s! [snapperd:16153]
> CPU: 0 PID: 16153 Comm: snapperd Not tainted 5.2.9-1-default #1 openSUSE Tumbleweed (unreleased)
> Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
> pstate: 40000005 (nZcv daif -PAN -UAO)
> pc : __ll_sc_arch_atomic_sub_return+0x14/0x20
> lr : btrfs_release_extent_buffer_pages+0xe0/0x1e8 [btrfs]
> sp : ffff00001273b7e0
> Call trace:
>  __ll_sc_arch_atomic_sub_return+0x14/0x20
>  release_extent_buffer+0xdc/0x120 [btrfs]
>  free_extent_buffer.part.0+0xb0/0x118 [btrfs]
>  free_extent_buffer+0x24/0x30 [btrfs]
>  btrfs_release_path+0x4c/0xa0 [btrfs]
>  btrfs_free_path.part.0+0x20/0x40 [btrfs]
>  btrfs_free_path+0x24/0x30 [btrfs]
>  get_inode_info+0xa8/0xf8 [btrfs]
>  finish_inode_if_needed+0xe0/0x6d8 [btrfs]
>  changed_cb+0x9c/0x410 [btrfs]
>  btrfs_compare_trees+0x284/0x648 [btrfs]
>  send_subvol+0x33c/0x520 [btrfs]
>  btrfs_ioctl_send+0x8a0/0xaf0 [btrfs]
>  btrfs_ioctl+0x199c/0x2288 [btrfs]
>  do_vfs_ioctl+0x4b0/0x820
>  ksys_ioctl+0x84/0xb8
>  __arm64_sys_ioctl+0x28/0x38
>  el0_svc_common.constprop.0+0x7c/0x188
>  el0_svc_handler+0x34/0x90
>  el0_svc+0x8/0xc
> 
> Fix this by adding a call to cond_resched at the beginning of the main
> loop in btrfs_compare_trees.
> 
> Fixes: 7069830a9e38 ("Btrfs: add btrfs_compare_trees function")
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>

Reviewed-by: David Sterba <dsterba@suse.com>