Re: [PATCH] io-wq: remove GFP_ATOMIC allocation off schedule out path

[Jens Axboe]

On 8/4/21 9:33 AM, Daniel Wagner wrote:
> On Wed, Aug 04, 2021 at 08:43:43AM -0600, Jens Axboe wrote:
>> Daniel reports that the v5.14-rc4-rt4 kernel throws a BUG when running
>> stress-ng:
>>
>> | [   90.202543] BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:35
>> | [   90.202549] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 2047, name: iou-wrk-2041
>> | [   90.202555] CPU: 5 PID: 2047 Comm: iou-wrk-2041 Tainted: G        W         5.14.0-rc4-rt4+ #89
>> | [   90.202559] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/2014
>> | [   90.202561] Call Trace:
>> | [   90.202577]  dump_stack_lvl+0x34/0x44
>> | [   90.202584]  ___might_sleep.cold+0x87/0x94
>> | [   90.202588]  rt_spin_lock+0x19/0x70
>> | [   90.202593]  ___slab_alloc+0xcb/0x7d0
>> | [   90.202598]  ? newidle_balance.constprop.0+0xf5/0x3b0
>> | [   90.202603]  ? dequeue_entity+0xc3/0x290
>> | [   90.202605]  ? io_wqe_dec_running.isra.0+0x98/0xe0
>> | [   90.202610]  ? pick_next_task_fair+0xb9/0x330
>> | [   90.202612]  ? __schedule+0x670/0x1410
>> | [   90.202615]  ? io_wqe_dec_running.isra.0+0x98/0xe0
>> | [   90.202618]  kmem_cache_alloc_trace+0x79/0x1f0
>> | [   90.202621]  io_wqe_dec_running.isra.0+0x98/0xe0
>> | [   90.202625]  io_wq_worker_sleeping+0x37/0x50
>> | [   90.202628]  schedule+0x30/0xd0
>> | [   90.202630]  schedule_timeout+0x8f/0x1a0
>> | [   90.202634]  ? __bpf_trace_tick_stop+0x10/0x10
>> | [   90.202637]  io_wqe_worker+0xfd/0x320
>> | [   90.202641]  ? finish_task_switch.isra.0+0xd3/0x290
>> | [   90.202644]  ? io_worker_handle_work+0x670/0x670
>> | [   90.202646]  ? io_worker_handle_work+0x670/0x670
>> | [   90.202649]  ret_from_fork+0x22/0x30
>>
>> which is due to the RT kernel not liking a GFP_ATOMIC allocation inside
>> a raw spinlock. Besides that not working on RT, doing any kind of
>> allocation from inside schedule() is kind of nasty and should be avoided
>> if at all possible.
>>
>> This particular path happens when an io-wq worker goes to sleep, and we
>> need a new worker to handle pending work. We currently allocate a small
>> data item to hold the information we need to create a new worker, but we
>> can instead include this data in the io_worker struct itself and just
>> protect it with a single bit lock. We only really need one per worker
>> anyway, as we will have run pending work between to sleep cycles.
>>
>> https://lore.kernel.org/lkml/20210804082418.fbibprcwtzyt5qax@xxxxxxxxxxxxx/
>> Reported-by: Daniel Wagner <dwagner@xxxxxxx>
>> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> 
> I applied this patch on top of v5.14-rc4-rt4 and with it all looks
> good.
> 
> Tested-by: Daniel Wagner <dwagner@xxxxxxx>

Great, thanks for testing!

-- 
Jens Axboe