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Subject[RFC PATCH] blk-cgroup: prevent rcu_sched detected stalls warnings in blkg_destroy_all()
Date
test procedures:

a. create 20000 cgroups, and echo "8:0 10000" to
blkio.throttle.write_bps_device
b. echo 1 > /sys/blocd/sda/device/delete

test result:

CPU: 6 PID: 472 Comm: bash Not tainted 5.10.0-rc4-next-20201120-dirty #62
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-p4
RIP: 0010:css_next_descendant_post+0x5d/0x110
Code: 00 48 8b b3 c0 00 00 00 48 89 df e8 1d e9 ff ff 49 89 c4 48 85 c0 74 68 49 8d 5c f
RSP: 0018:ffff88810761faf8 EFLAGS: 00000046
RAX: ffff888001f1a830 RBX: ffff888001f1a830 RCX: ffffffff812e0548
RDX: dffffc0000000000 RSI: ffff888106f3f400 RDI: ffff888001f1a830
RBP: ffff8880133b14c0 R08: ffffffff812dee71 R09: 0000000000000001
R10: 0000000000000003 R11: ffffed1020ec3f6b R12: ffff888001f1a800
R13: ffff888100ff58b4 R14: 0000000000000016 R15: ffffffff8c330180
FS: 00007f36ed4df700(0000) GS:ffff88810b380000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055c8a5a45010 CR3: 000000000436c000 CR4: 00000000000006e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
blk_throtl_update_limit_valid.isra.0+0x193/0x2b0
throtl_pd_offline+0x95/0x100
blkg_destroy+0xb6/0x2b0
blkg_destroy_all+0x9f/0x140
blkcg_exit_queue+0x16/0x30
blk_release_queue+0x17e/0x320
kobject_put+0x1b2/0x440
blk_put_queue+0x16/0x20
scsi_device_dev_release_usercontext+0x38c/0x5f0
? scsi_device_cls_release+0x30/0x30
execute_in_process_context+0x2d/0xb0
scsi_device_dev_release+0x20/0x30
device_release+0xbc/0x180
kobject_put+0x1b2/0x440
put_device+0x17/0x30
scsi_device_put+0x52/0x60
sdev_store_delete+0x93/0x100
? dev_driver_string+0xa0/0xa0
dev_attr_store+0x40/0x70
sysfs_kf_write+0x89/0xc0
kernfs_fop_write+0x167/0x310
? sysfs_kf_bin_read+0x130/0x130
vfs_write+0x104/0x4c0
ksys_write+0xcd/0x1e0
? __x64_sys_read+0x60/0x60
? up_read+0x25/0xf0
? do_user_addr_fault+0x408/0x990
__x64_sys_write+0x46/0x60
do_syscall_64+0x45/0x70
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7f36ecbcd130
Code: 73 01 c3 48 8b 0d 58 ed 2c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 83 4
RSP: 002b:00007ffe261fe678 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00007f36ecbcd130
RDX: 0000000000000002 RSI: 0000559fb4244080 RDI: 0000000000000001
RBP: 0000559fb4244080 R08: 000000000000000a R09: 00007f36ed4df700
R10: 0000559fb45d0180 R11: 0000000000000246 R12: 0000000000000002
R13: 0000000000000001 R14: 00007f36ece9d5e0 R15: 00007f36ece988c0

The usage of so many blkg is very rare, however, such problem do exist
in theory. In order to avoid such warnings, release 'q->queue_lock' for
a while when a batch of blkg were destroyed.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
---
block/blk-cgroup.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index c68bdf58c9a6..566195490f4a 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -420,14 +420,27 @@ static void blkg_destroy(struct blkcg_gq *blkg)
*
* Destroy all blkgs associated with @q.
*/
+#define BLKG_DESTROY_BATH 4096
static void blkg_destroy_all(struct request_queue *q)
{
struct blkcg_gq *blkg, *n;
+ int count = BLKG_DESTROY_BATH;

spin_lock_irq(&q->queue_lock);
list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
struct blkcg *blkcg = blkg->blkcg;

+ /*
+ * If the list is too long, the loop can took a long time,
+ * thus relese the lock for a while when a batch of blkcg
+ * were destroyed.
+ */
+ if (!(--count)) {
+ count = BLKG_DESTROY_BATH;
+ spin_unlock_irq(&q->queue_lock);
+ cond_resched();
+ spin_lock_irq(&q->queue_lock);
+ }
spin_lock(&blkcg->lock);
blkg_destroy(blkg);
spin_unlock(&blkcg->lock);
--
2.25.4
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