Re: [PATCH v2 3/8] mm: Optimise madvise WILLNEED

[Qian Cai]

On Mon, 2020-09-14 at 12:17 -0400, Qian Cai wrote:
> On Thu, 2020-09-10 at 19:33 +0100, Matthew Wilcox (Oracle) wrote:
> > Instead of calling find_get_entry() for every page index, use an XArray
> > iterator to skip over NULL entries, and avoid calling get_page(),
> > because we only want the swap entries.
> > 
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> 
> Reverting the "Return head pages from find_*_entry" patchset [1] up to this
> patch fixed the issue that LTP madvise06 test [2] would trigger endless soft-
> lockups below. It does not help after applied patches fixed other separate
> issues in the patchset [3][4].

Forgot to send this piece of RCU stall traces as well which might help
debugging.

00: [ 2852.137748] madvise06 (62712): drop_caches: 3                            
01: [ 2928.208367] rcu: INFO: rcu_sched self-detected stall on CPU              
01: [ 2928.210083] rcu:     1-....: (6499 ticks this GP) idle=036/1/0x4000000000
01: 000002 softirq=1741392/1741392 fqs=3161                                     
01: [ 2928.210610]  (t=6500 jiffies g=610849 q=12529)                           
01: [ 2928.210620] Task dump for CPU 1:                                         
01: [ 2928.210630] task:madvise06       state:R  running task     stack:53320 pi
01: d:62712 ppid: 62711 flags:0x00000004                                        
01: [ 2928.210676] Call Trace:                                                  
01: [ 2928.210693]  [<00000000af57ec88>] show_stack+0x158/0x1f0                 
01: [ 2928.210703]  [<00000000ae55b692>] sched_show_task+0x3d2/0x4c8            
01: [ 2928.210710]  [<00000000af5846aa>] rcu_dump_cpu_stacks+0x26a/0x2a8        
01: [ 2928.210718]  [<00000000ae64fa62>] rcu_sched_clock_irq+0x1c92/0x2188      
01: [ 2928.210726]  [<00000000ae6662ee>] update_process_times+0x4e/0x148        
01: [ 2928.210734]  [<00000000ae690c26>] tick_sched_timer+0x86/0x188            
01: [ 2928.210741]  [<00000000ae66989c>] __hrtimer_run_queues+0x84c/0x10b8      
01: [ 2928.210748]  [<00000000ae66c80a>] hrtimer_interrupt+0x38a/0x860          
01: [ 2928.210758]  [<00000000ae48dbf2>] do_IRQ+0x152/0x1c8                     
01: [ 2928.210767]  [<00000000af5b00ea>] ext_int_handler+0x18e/0x194            
01: [ 2928.210774]  [<00000000ae5e332e>] arch_local_irq_restore+0x86/0xa0       
01: [ 2928.210782]  [<00000000af58da04>] lock_is_held_type+0xe4/0x130           
01: [ 2928.210791]  [<00000000ae63355a>] rcu_read_lock_held+0xba/0xd8           
01: [ 2928.210799]  [<00000000af0125fc>] xas_descend+0x244/0x2c8                
01: [ 2928.210806]  [<00000000af012754>] xas_load+0xd4/0x148                    
01: [ 2928.210812]  [<00000000af014490>] xas_find+0x5d0/0x818                   
01: [ 2928.210822]  [<00000000ae97e644>] do_madvise+0xd5c/0x1600                
01: [ 2928.210828]  [<00000000ae97f2d2>] __s390x_sys_madvise+0x72/0x98          
01: [ 2928.210835]  [<00000000af5af844>] system_call+0xdc/0x278                 
01: [ 2928.210841] 3 locks held by madvise06/62712:                             
01: [ 2928.216406]  #0: 00000001437fca18 (&mm->mmap_lock){++++}-{3:3}, at: do_m 
01: dvise+0x18c/0x1600                                                          
01: [ 2928.216430]  #1: 00000000afbdd3e0 (rcu_read_lock){....}-{1:2}, at: do_mad
01: vise+0xe72/0x1600                                                           
01: [ 2928.216449]  #2: 00000000afbe0818 (rcu_node_1){-.-.}-{2:2}, at: rcu_dump_
01: cpu_stacks+0xb2/0x2a8