Re: RCU stall when using function_graph

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Aug 01, 2017 at 03:04:05PM -0700, Paul E. McKenney wrote:
> > Hi Paul,
> > 
> > I have been trying to set the function_graph tracer for ftrace and each time I
> > get a CPU stall.
> > 
> > How to reproduce:
> > -----------------
> > 
> > 		 echo function_graph > /sys/kernel/debug/tracing/current_tracer
> > 
> > This error appears with v4.13-rc3 and v4.12-rc6.
> > 
> > Is it something already reported ?
> 
> I have seen this sort of thing, but only when actually dumping the trace
> out, and I though those got fixed.  You are seeing this just accumulating
> the trace?
> 
> These RCU CPU stall warnings usually occur when something grabs hold of
> a CPU for too long, as in 21 seconds or so.  One way that they can happen
> is excessive lock contention, another is having the kernel run through
> too much data at one shot.
> 
> Adding Steven Rostedt on CC for his thoughts.

And Peter Zijlstra suggested this in response to a similar splat:

https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=locking/core&id=15377ef4fe0c86eb7fa1099575b2e86357d99e42

							Thanx, Paul

> > [ ... ]
> > 
> > [  472.934986] INFO: rcu_preempt detected stalls on CPUs/tasks:
> > [  472.941302]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=3195
> > [  472.948982]  (detected by 3, t=6502 jiffies, g=549, c=548, q=10)
> > [  472.955180] Task dump for CPU 0:
> > [  472.958513] swapper/0       R  running task        0     0      0 0x00000002
> > [  550.954786] INFO: rcu_preempt detected stalls on CPUs/tasks:
> > [  550.960657]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=12926
> > [  550.967897]  (detected by 3, t=26007 jiffies, g=549, c=548, q=27)
> > [  550.974181] Task dump for CPU 0:
> > [  550.977513] swapper/0       R  running task        0     0      0 0x00000002
> > [  628.974788] INFO: rcu_preempt detected stalls on CPUs/tasks:
> > [  628.980659]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=22666
> > [  628.987900]  (detected by 3, t=45512 jiffies, g=549, c=548, q=43)
> > [  628.994184] Task dump for CPU 0:
> > [  628.997518] swapper/0       R  running task        0     0      0 0x00000002
> > [  706.994787] INFO: rcu_preempt detected stalls on CPUs/tasks:
> > [  707.000658]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=31785
> > [  707.007899]  (detected by 3, t=65017 jiffies, g=549, c=548, q=74)
> > [  707.014183] Task dump for CPU 0:
> > [  707.017516] swapper/0       R  running task        0     0      0 0x00000002
> > [  707.024902] rcu_preempt kthread starved for 1261 jiffies! g549 c548 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
> > [  707.034548] rcu_preempt     S    0     8      2 0x00000000
> > [  726.049583] INFO: task jbd2/mmcblk1p2-:1120 blocked for more than 120 seconds.
> > [  726.061772]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
> > [  726.074294] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [  726.089027] jbd2/mmcblk1p2- D    0  1120      2 0x00000000
> > [  726.126737] INFO: task rs:main Q:Reg:2272 blocked for more than 120 seconds.
> > [  726.140484]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
> > [  726.154515] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [  726.170140] rs:main Q:Reg   D    0  2272      1 0x00000000
> > [  726.205626]
> > [  726.205626] Showing all locks held in the system:
> > [  726.272952]
> > [  726.278194] =============================================
> > [  726.278194]
> > [  785.014779] INFO: rcu_preempt detected stalls on CPUs/tasks:
> > [  785.020658]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=41387
> > [  785.027905]  (detected by 3, t=84524 jiffies, g=549, c=548, q=94)
> > [  785.034194] Task dump for CPU 0:
> > [  785.037532] swapper/0       R  running task        0     0      0 0x00000002
> > [  846.879753] INFO: task jbd2/mmcblk1p2-:1120 blocked for more than 120 seconds.
> > [  846.892966]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
> > [  846.909597] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [  846.926726] jbd2/mmcblk1p2- D    0  1120      2 0x00000000
> > [  846.981356] INFO: task rs:main Q:Reg:2272 blocked for more than 120 seconds.
> > [  847.001593]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
> > [  847.017642] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [  847.034135] rs:main Q:Reg   D    0  2272      1 0x00000000
> > [  847.077209]
> > [  847.077209] Showing all locks held in the system:
> > [  847.180736]
> > [  847.186753] =============================================
> > [  847.186753]
> > [  863.034778] INFO: rcu_preempt detected stalls on CPUs/tasks:
> > [  863.040660]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=50881
> > [  863.047907]  (detected by 4, t=104030 jiffies, g=549, c=548, q=114)
> > [  863.054373] Task dump for CPU 0:
> > [  863.057711] swapper/0       R  running task        0     0      0 0x00000002
> > [  941.054780] INFO: rcu_preempt detected stalls on CPUs/tasks:
> > [  941.060659]  0-...: (3 GPs behind) idle=d1a/2/0 softirq=2556/2557 fqs=59726
> > [  941.067903]  (detected by 7, t=123535 jiffies, g=549, c=548, q=303)
> > [  941.074370] Task dump for CPU 0:
> > [  941.077708] swapper/0       R  running task        0     0      0 0x00000002
> > [  967.711682] INFO: task jbd2/mmcblk1p2-:1120 blocked for more than 120 seconds.
> > [  967.725731]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
> > [  967.743347] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [  967.765658] jbd2/mmcblk1p2- D    0  1120      2 0x00000000
> > [  967.810595] INFO: task rs:main Q:Reg:2272 blocked for more than 120 seconds.
> > [  967.825360]       Not tainted 4.13.0-rc3-ktest-linaro-hikey #202
> > [  967.838595] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > [  967.854460] rs:main Q:Reg   D    0  2272      1 0x00000000
> > [  967.897804]
> > [  967.897804] Showing all locks held in the system:
> > [  967.997067]
> > [  968.002570] =============================================
> > 
> > 
> > [ ... ]
> > 
> > etc ...