Commit e1333462 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

doc: Update RCU CPU stall-warning documentation 



Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 869cc745

Documentation/RCU/stallwarn.txt

+12 −12
Original line number Diff line number Diff line
@@ -172,7 +172,7 @@ it will print a message similar to the following: 
	INFO: rcu_sched detected stalls on CPUs/tasks:

	2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0

	16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0

	(detected by 32, t=2603 jiffies, g=7073, c=7072, q=625)

	(detected by 32, t=2603 jiffies, g=7075, q=625)



This message indicates that CPU 32 detected that CPUs 2 and 16 were both

causing stalls, and that the stall was affecting RCU-sched.  This message
@@ -215,11 +215,10 @@ CPU since the last time that this CPU noted the beginning of a grace 
period.



The "detected by" line indicates which CPU detected the stall (in this

case, CPU 32), how many jiffies have elapsed since the start of the

grace period (in this case 2603), the number of the last grace period

to start and to complete (7073 and 7072, respectively), and an estimate

of the total number of RCU callbacks queued across all CPUs (625 in

this case).

case, CPU 32), how many jiffies have elapsed since the start of the grace

period (in this case 2603), the grace-period sequence number (7075), and

an estimate of the total number of RCU callbacks queued across all CPUs

(625 in this case).



In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed

for each CPU:
@@ -266,15 +265,16 @@ If the relevant grace-period kthread has been unable to run prior to 
the stall warning, as was the case in the "All QSes seen" line above,

the following additional line is printed:



	kthread starved for 23807 jiffies! g7073 c7072 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1

	kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5



Starving the grace-period kthreads of CPU time can of course result

in RCU CPU stall warnings even when all CPUs and tasks have passed

through the required quiescent states.  The "g" and "c" numbers flag the

number of the last grace period started and completed, respectively,

the "f" precedes the ->gp_flags command to the grace-period kthread,

the "RCU_GP_WAIT_FQS" indicates that the kthread is waiting for a short

timeout, and the "state" precedes value of the task_struct ->state field.

through the required quiescent states.  The "g" number shows the current

grace-period sequence number, the "f" precedes the ->gp_flags command

to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the

kthread is waiting for a short timeout, the "state" precedes value of the

task_struct ->state field, and the "cpu" indicates that the grace-period

kthread last ran on CPU 5.





Multiple Warnings From One Stall