[PATCH] mm: make mm_percpu_wq non freezable

[Michal Hocko]

Geert has reported a freeze during PM resume and some additional
debugging has shown that the device_resume worker cannot make a forward
progress because it waits for an event which is stuck waiting in
drain_all_pages:
[  243.691979] INFO: task kworker/u4:0:5 blocked for more than 120 seconds.
[  243.698684]       Not tainted 4.11.0-rc7-koelsch-00029-g005882e53d62f25d-dirty #3476
[  243.706439] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  243.714276] kworker/u4:0    D    0     5      2 0x00000000
[  243.719780] Workqueue: events_unbound async_run_entry_fn
[  243.725118] [<c0700c20>] (__schedule) from [<c0700f44>] (schedule+0xb0/0xcc)
[  243.732181] [<c0700f44>] (schedule) from [<c0705108>] (schedule_timeout+0x18/0x1f4)
[  243.739840] [<c0705108>] (schedule_timeout) from [<c07019c0>] (wait_for_common+0x100/0x19c)
[  243.748207] [<c07019c0>] (wait_for_common) from [<c04d2008>] (dpm_wait_for_superior+0x14/0x5c)
[  243.756836] [<c04d2008>] (dpm_wait_for_superior) from [<c04d2624>] (device_resume+0x40/0x1a0)
[  243.765380] [<c04d2624>] (device_resume) from [<c04d279c>] (async_resume+0x18/0x44)
[  243.773055] [<c04d279c>] (async_resume) from [<c023db24>] (async_run_entry_fn+0x44/0x114)
[  243.781245] [<c023db24>] (async_run_entry_fn) from [<c0236534>] (process_one_work+0x1cc/0x31c)
[  243.789876] [<c0236534>] (process_one_work) from [<c0236c90>] (worker_thread+0x2b8/0x3f0)
[  243.798080] [<c0236c90>] (worker_thread) from [<c023b230>] (kthread+0x120/0x140)
[  243.805500] [<c023b230>] (kthread) from [<c0206d68>] (ret_from_fork+0x14/0x2c)
[...]
[  249.441198] bash            D    0  1703   1694 0x00000000
[  249.446702] [<c0700c20>] (__schedule) from [<c0700f44>] (schedule+0xb0/0xcc)
[  249.453764] [<c0700f44>] (schedule) from [<c0705108>] (schedule_timeout+0x18/0x1f4)
[  249.461427] [<c0705108>] (schedule_timeout) from [<c07019c0>] (wait_for_common+0x100/0x19c)
[  249.469797] [<c07019c0>] (wait_for_common) from [<c0234e44>] (flush_work+0x128/0x158)
[  249.477650] [<c0234e44>] (flush_work) from [<c02ab488>] (drain_all_pages+0x198/0x1f0)
[  249.485503] [<c02ab488>] (drain_all_pages) from [<c02e1a1c>] (start_isolate_page_range+0xd8/0x1ac)
[  249.494484] [<c02e1a1c>] (start_isolate_page_range) from [<c02ae464>] (alloc_contig_range+0xc4/0x304)
[  249.503724] [<c02ae464>] (alloc_contig_range) from [<c02e1e78>] (cma_alloc+0x134/0x1bc)
[  249.511739] [<c02e1e78>] (cma_alloc) from [<c021308c>] (__alloc_from_contiguous+0x30/0xa0)
[  249.520023] [<c021308c>] (__alloc_from_contiguous) from [<c021313c>] (cma_allocator_alloc+0x40/0x48)
[  249.529173] [<c021313c>] (cma_allocator_alloc) from [<c0213318>] (__dma_alloc+0x1d4/0x2e8)
[  249.537455] [<c0213318>] (__dma_alloc) from [<c02134a8>] (arm_dma_alloc+0x40/0x4c)
[  249.545047] [<c02134a8>] (arm_dma_alloc) from [<c0534548>] (sh_eth_ring_init+0xec/0x1b8)
[  249.553160] [<c0534548>] (sh_eth_ring_init) from [<c0536df0>] (sh_eth_open+0x88/0x1e0)
[  249.561086] [<c0536df0>] (sh_eth_open) from [<c0536fc4>] (sh_eth_resume+0x7c/0xc0)
[  249.568678] [<c0536fc4>] (sh_eth_resume) from [<c04d2240>] (dpm_run_callback+0x48/0xc8)
[  249.576702] [<c04d2240>] (dpm_run_callback) from [<c04d2740>] (device_resume+0x15c/0x1a0)
[  249.584898] [<c04d2740>] (device_resume) from [<c04d3644>] (dpm_resume+0xe4/0x244)
[  249.592485] [<c04d3644>] (dpm_resume) from [<c04d3968>] (dpm_resume_end+0xc/0x18)
[  249.599977] [<c04d3968>] (dpm_resume_end) from [<c0261010>] (suspend_devices_and_enter+0x3c8/0x490)
[  249.609042] [<c0261010>] (suspend_devices_and_enter) from [<c0261300>] (pm_suspend+0x228/0x280)
[  249.617759] [<c0261300>] (pm_suspend) from [<c025fecc>] (state_store+0xac/0xcc)
[  249.625089] [<c025fecc>] (state_store) from [<c0343b04>] (kernfs_fop_write+0x164/0x1a0)
[  249.633116] [<c0343b04>] (kernfs_fop_write) from [<c02e5838>] (__vfs_write+0x20/0x108)
[  249.641043] [<c02e5838>] (__vfs_write) from [<c02e6c08>] (vfs_write+0xb8/0x144)
[  249.648373] [<c02e6c08>] (vfs_write) from [<c02e788c>] (SyS_write+0x40/0x80)
[  249.655437] [<c02e788c>] (SyS_write) from [<c0206cc0>] (ret_fast_syscall+0x0/0x34)
[...]
[  254.753928] Showing busy workqueues and worker pools:
[...]
[  254.854225] workqueue mm_percpu_wq: flags=0xc
[  254.858583]   pwq 2: cpus=1 node=0 flags=0x0 nice=0 active=0/0
[  254.864428]     delayed: drain_local_pages_wq, vmstat_update
[  254.870111]   pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=0/0
[  254.875957]     delayed: drain_local_pages_wq BAR(1703), vmstat_update

Tetsuo has properly noted that mm_percpu_wq is created as WQ_FREEZABLE
so it is frozen this early during resume so we are effectively deadlocked.
Fix this by dropping WQ_FREEZABLE when creating mm_percpu_wq. We really want to
have it operational all the time.

Fixes: ce612879ddc7 ("mm: move pcp and lru-pcp draining into single wq")
Reported-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Debugged-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/vmstat.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 757be8303aa0..f5fa1bd1eb16 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1766,8 +1766,7 @@ void __init init_mm_internals(void)
 {
 	int ret __maybe_unused;
 
-	mm_percpu_wq = alloc_workqueue("mm_percpu_wq",
-				       WQ_FREEZABLE|WQ_MEM_RECLAIM, 0);
+	mm_percpu_wq = alloc_workqueue("mm_percpu_wq", WQ_MEM_RECLAIM, 0);
 
 #ifdef CONFIG_SMP
 	ret = cpuhp_setup_state_nocalls(CPUHP_MM_VMSTAT_DEAD, "mm/vmstat:dead",
-- 
2.11.0

-- 
Michal Hocko
SUSE Labs