path: root/Documentation/RCU
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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-12-11 10:20:59 -0800
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2015-01-06 11:05:27 -0800
commit6ccd2ecd422644277b7d8b37222e3af3f43ea9ae (patch)
tree7aabe254edd3dc05e992d631123a3275ef4dd1f6 /Documentation/RCU
parentfc908ed33e7c1428f799abb12399f906da03b397 (diff)
rcu: Improve diagnostics for spurious RCU CPU stall warnings
The current RCU CPU stall warning code will print "Stall ended before state dump start" any time that the stall-warning code is triggered on a CPU that has already reported a quiescent state for the current grace period and if all quiescent states have been reported for the current grace period. However, a true stall can result in these symptoms, for example, by preventing RCU's grace-period kthreads from ever running This commit therefore checks for this condition, reporting the end of the stall only if one of the grace-period counters has actually advanced. Otherwise, it reports the last time that the grace-period kthread made meaningful progress. (In normal situations, the grace-period kthread should make meaningful progress at least every jiffies_till_next_fqs jiffies.) Reported-by: Miroslav Benes <mbenes@suse.cz> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Miroslav Benes <mbenes@suse.cz>
Diffstat (limited to 'Documentation/RCU')
1 files changed, 5 insertions, 0 deletions
diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index ed186a902d31..55f9707fe60a 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -187,6 +187,11 @@ o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the
behavior, you might need to replace some of the cond_resched()
calls with calls to cond_resched_rcu_qs().
+o Anything that prevents RCU's grace-period kthreads from running.
+ This can result in the "All QSes seen" console-log message.
+ This message will include information on when the kthread last
+ ran and how often it should be expected to run.
o A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
happen to preempt a low-priority task in the middle of an RCU
read-side critical section. This is especially damaging if

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