Bug #13096
closed
xnf asleep at wheel while freemem smashes into the ground
100%
Description
The xnf driver (for Xen virtual network interfaces) performs a variety of exciting allocations within xnf_send(). When the system is under memory pressure, those allocations can block waiting for pages to become available. Also while the system is under memory pressure, various kmem caches need to be reaped to free up pages; at least one of those (ip_ire_reclaim()) appears to need to execute in an squeue. The reap can get stuck waiting to enter the squeue forever, preventing any subsequent reaping and all but ensuring deadlock.
In a dump from the firing of the pageout deadman, we can see the kmem_taskq is active and about half way through a reap:
> ::taskq -n kmem_taskq ADDR NAME ACT/THDS Q'ED MAXQ INST fffffe0bd3416c68 kmem_taskq 1/ 1 236 482 0
It is stuck in ip_ire_reclaim():
> ::taskq -n kmem_taskq | ::walk taskq_thread | ::stacks
THREAD STATE SOBJ COUNT
fffffe000f417c20 SLEEP CV 1
swtch+0x133
cv_wait+0x68
tcp_ixa_cleanup_wait_and_finish+0x53
conn_ixa_cleanup+0x1a3
ipcl_walk+0x91
ip_ire_reclaim_stack+0x56
ip_ire_reclaim+0x36
kmem_cache_reap+0x3e
taskq_thread+0x2cd
thread_start+0xb
The conn_ixa_cleanup() routine kicks some work into an squeue and waits for it to complete. Where are we on squeues:
> ::squeue -v
ADDR STATE CPU FIRST LAST WORKER
fffffe0bd3d05d00 00329 1 fffffe0bdceede60 fffffe0bdecd81c0 fffffe0010bd7c20
|
+--> SQS_PROC being processed
SQS_FAST ... in fast-path mode
SQS_BOUND worker thread bound to CPU
ADDR STATE CPU FIRST LAST WORKER
fffffe0bd3d05dc0 00820 1 0000000000000000 0000000000000000 fffffe000fcc5c20
|
+--> SQS_BOUND worker thread bound to CPU
ADDR STATE CPU FIRST LAST WORKER
fffffe0bd3d05e80 00820 0 0000000000000000 0000000000000000 fffffe000f58bc20
|
+--> SQS_BOUND worker thread bound to CPU
Ah, one is running. Let's see what it is doing:
> fffffe0bd3d05d00::print squeue_t sq_run | ::stacks
THREAD STATE SOBJ COUNT
fffffe0bdd9920c0 SLEEP CV 1
swtch+0x133
cv_wait+0x68
page_create_throttle+0x174
page_create_va+0x598
segkmem_page_create+0x97
segkmem_xalloc+0x13f
segkmem_alloc_vn+0x3b
segkmem_alloc+0x17
vmem_xalloc+0x629
vmem_alloc+0x190
kmem_slab_create+0x7c
kmem_slab_alloc+0x10b
kmem_cache_alloc+0x15b
xnf_data_txbuf_alloc+0x17
xnf_mblk_map+0x69
xnf_send+0x23f
mac_ring_tx+0x43
mac_provider_tx+0x85
mac_tx+0x295
str_mdata_fastpath_put+0x8e
ip_xmit+0x843
ire_send_wire_v4+0x345
conn_ip_output+0x1d4
tcp_send_data+0x58
tcp_output+0x582
squeue_enter+0x3f9
tcp_sendmsg+0x16c
so_sendmsg+0x24a
socket_sendmsg+0x62
socket_vop_write+0x61
fop_write+0x60
write+0x2c6
Ah.
Updated by Joshua M. Clulow over 1 year ago
Testing Notes¶
I built an AMI build from OpenIndiana plus the RTI build bits. I ran iperf between the test guest built from the new AMI, and an existing instance running other bits. Networking appeared stable, so I tried running the machine out of memory. If I killed the test early enough, after creating some but not terminally sustained pressure, the networking stack seemed to be able to continue to function. Because not all of the pageout fixes are in, after sustained paging activity the machine deadlocked, but the networking stuff did not appear to be the cause.
Updated by Joshua M. Clulow over 1 year ago
To fix this, I changed allocations for buffers in the data path from sleeping (KM_SLEEP) to non-sleeping (KM_NOSLEEP) so that they will ultimately fail and return NULL when there are no pages readily available instead of blocking. The TX paths that use these allocation functions already have error return paths which appear relatively sound, so we will now exercise those paths on allocation failures as well. Initial allocations such as when the driver is starting up are still sleeping allocations.
Updated by Electric Monk over 1 year ago
- Status changed from New to Closed
- % Done changed from 0 to 100
git commit cfd17c15945080ff766acfba4bfbc0ac4d2d31cd
commit cfd17c15945080ff766acfba4bfbc0ac4d2d31cd
Author: Joshua M. Clulow <josh@sysmgr.org>
Date: 2020-09-23T22:22:51.000Z
13096 xnf asleep at wheel while freemem smashes into the ground
Reviewed by: Jason King <jason.brian.king@gmail.com>
Approved by: Robert Mustacchi <rm@fingolfin.org>