srcu: Specify auto-expedite holdoff time

On small systems, in the absence of readers, expedited SRCU grace
periods can complete in less than a microsecond.  This means that an
eight-CPU system can have all CPUs doing synchronize_srcu() in a tight
loop and almost always expedite.  This might actually be desirable in
some situations, but in general it is a good way to needlessly burn
CPU cycles.  And in those situations where it is desirable, your friend
is the function synchronize_srcu_expedited().

For other situations, this commit adds a kernel parameter that specifies
a holdoff between completing the last SRCU grace period and auto-expediting
the next.  If the next grace period starts before the holdoff expires,
auto-expediting is disabled.  The holdoff is 50 microseconds by default,
and can be tuned to the desired number of nanoseconds.  A value of zero
disables auto-expediting.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Mike Galbraith <efault@gmx.de>

1 files changed, 8 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index facc20a3f962..4a4b9266c4de 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3779,6 +3779,14 @@
 	spia_pedr=
 	spia_peddr=
 
+	srcutree.exp_holdoff [KNL]
+			Specifies how many nanoseconds must elapse
+			since the end of the last SRCU grace period for
+			a given srcu_struct until the next normal SRCU
+			grace period will be considered for automatic
+			expediting.  Set to zero to disable automatic
+			expediting.
+
 	stacktrace	[FTRACE]
 			Enabled the stack tracer on boot up.