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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-05-03 15:35:32 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2017-06-08 18:52:34 -0700
commitc350c008297643dad3c395c2fd92230142da5cf6 (patch)
tree731bff2816d661ee0c1015a6e5e4423ccfd87f7f /Documentation
parent71c40fd0b5ceb300c6cb8753835d9d94a8bfc56f (diff)
srcu: Prevent sdp->srcu_gp_seq_needed counter wrap
If a given CPU never happens to ever start an SRCU grace period, the grace-period sequence counter might wrap. If this CPU were to decide to finally start a grace period, the state of its sdp->srcu_gp_seq_needed might make it appear that it has already requested this grace period, which would prevent starting the grace period. If no other CPU ever started a grace period again, this would look like a grace-period hang. Even if some other CPU took pity and started the needed grace period, the leaf rcu_node structure's ->srcu_data_have_cbs field won't have record of the fact that this CPU has a callback pending, which would look like a very localized grace-period hang. This might seem very unlikely, but SRCU grace periods can take less than a microsecond on small systems, which means that overflow can happen in much less than an hour on a 32-bit embedded system. And embedded systems are especially likely to have long-term idle CPUs. Therefore, it makes sense to prevent this scenario from happening. This commit therefore scans each srcu_data structure occasionally, with frequency controlled by the srcutree.counter_wrap_check kernel boot parameter. This parameter can be set to something like 255 in order to exercise the counter-wrap-prevention code. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/admin-guide/kernel-parameters.txt9
1 files changed, 9 insertions, 0 deletions
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 01b5ab92d251..6671f9b60a86 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3810,6 +3810,15 @@
spia_pedr=
spia_peddr=
+ srcutree.counter_wrap_check [KNL]
+ Specifies how frequently to check for
+ grace-period sequence counter wrap for the
+ srcu_data structure's ->srcu_gp_seq_needed field.
+ The greater the number of bits set in this kernel
+ parameter, the less frequently counter wrap will
+ be checked for. Note that the bottom two bits
+ are ignored.
+
srcutree.exp_holdoff [KNL]
Specifies how many nanoseconds must elapse
since the end of the last SRCU grace period for