Re: [PATCH] fs/fs-writeback.c: add a preemption point to move_expired_inodes

[Jan Kara <jack@suse.cz>]

On Wed 29-09-21 01:34:04, Wen Yang wrote:
> We encountered an unrecovered_softlockup issue on !PREEMPT
> kernel config with 4.9 based kernel.
> 
> PID: 185895  TASK: ffff880455dac280  CPU: 8   COMMAND: "kworker/u449:39"
>  #0 [ffff883f7e803c08] machine_kexec at ffffffff81061578
>  #1 [ffff883f7e803c68] __crash_kexec at ffffffff81127c19
>  #2 [ffff883f7e803d30] panic at ffffffff811b2255
>  #3 [ffff883f7e803db8] unrecovered_softlockup_detect at ffffffff811b2d57
>  #4 [ffff883f7e803ee0] watchdog_timer_fn at ffffffff8115827e
>  #5 [ffff883f7e803f18] __hrtimer_run_queues at ffffffff811085e3
>  #6 [ffff883f7e803f70] hrtimer_interrupt at ffffffff81108d8a
>  #7 [ffff883f7e803fc0] local_apic_timer_interrupt at ffffffff810580f8
>  #8 [ffff883f7e803fd8] smp_apic_timer_interrupt at ffffffff81745405
>  #9 [ffff883f7e803ff0] apic_timer_interrupt at ffffffff81743b90
>  --- <IRQ stack> ---
>  #10 [ffffc90086a93b88] apic_timer_interrupt at ffffffff81743b90
>     [exception RIP: __list_del_entry+44]
>     RIP: ffffffff813be22c  RSP: ffffc90086a93c30  RFLAGS: 00000202
>     RAX: ffff88522b8f8418  RBX: ffff88522b8f8418  RCX: dead000000000200
>     RDX: ffff8816fab00e68  RSI: ffffc90086a93c60  RDI: ffff8816fab01af8
>     RBP: ffffc90086a93c30   R8: ffff8816fab01af8   R9: 0000000100400018
>     R10: ffff885ae5ed8280  R11: 0000000000000000  R12: ffff8816fab01af8
>     R13: ffffc90086a93c60  R14: ffffc90086a93d08  R15: ffff883f631d2000
>     ORIG_RAX: ffffffffffffff10  CS: 0010  SS: 0000
>  #11 [ffffc90086a93c38] move_expired_inodes at ffffffff8127c74c
>  #12 [ffffc90086a93ca8] queue_io at ffffffff8127cde6
>  #13 [ffffc90086a93cd8] wb_writeback at ffffffff8128121f
>  #14 [ffffc90086a93d80] wb_workfn at ffffffff812819f4
>  #15 [ffffc90086a93e18] process_one_work at ffffffff810a5dc9
>  #16 [ffffc90086a93e60] worker_thread at ffffffff810a60ae
>  #17 [ffffc90086a93ec0] kthread at ffffffff810ac696
>  #18 [ffffc90086a93f50] ret_from_fork at ffffffff81741dd9
> 
> crash> set
>     PID: 185895
> COMMAND: "kworker/u449:39"
>    TASK: ffff880455dac280  [THREAD_INFO: ffff880455dac280]
>     CPU: 8
>   STATE: TASK_RUNNING (PANIC)
> 
> It has been running continuously for 53.052, as follows:
> crash> ps -m | grep 185895
> [  0 00:00:53.052] [RU]  PID: 185895  TASK: ffff880455dac280  CPU: 8
> COMMAND: "kworker/u449:39"
> 
> And the TIF_NEED_RESCHED flag has been set, as follows:
> crash> struct thread_info -x ffff880455dac280
> struct thread_info {
>   flags = 0x88,
>   status = 0x0
> }
> 
> Let's just add cond_resched() within move_expired_inodes()'s list-moving loop in
> order to avoid the watchdog splats.
> 
> Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  fs/fs-writeback.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 06d04a7..1546121 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1404,6 +1404,7 @@ static int move_expired_inodes(struct list_head *delaying_queue,
>  		if (sb && sb != inode->i_sb)
>  			do_sb_sort = 1;
>  		sb = inode->i_sb;
> +		cond_resched();

Thanks for the patch but you certainly cannot do this since we are holding
wb->list_lock during the whole move_expired_inodes() duration. It is not
trivial to implement safe dropping of the lock in move_expired_inodes() (or
queue_io() for that matter). How many inodes were there on b_dirty, b_io,
b_more_io, and tmp lists and from how many superblocks?

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR