From 09f501a0f0b5ad81f79049a31fb71f99f282d663 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 27 Apr 2017 13:16:15 -0700 Subject: [PATCH] srcu: Document auto-expediting requirement This commit documents the auto-expediting requirement satisfied by commits 2da4b2a7fd8d ("srcu: Expedite first synchronize_srcu() when idle") and 22607d66bbc3 ("srcu: Specify auto-expedite holdoff time"). Signed-off-by: Paul E. McKenney --- .../RCU/Design/Requirements/Requirements.html | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index f60adf112663..8bbf0bb18389 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -2935,6 +2935,20 @@ The reason that this is possible is that SRCU is insensitive to whether or not a CPU is online, which means that srcu_barrier() need not exclude CPU-hotplug operations. +

+SRCU also differs from other RCU flavors in that SRCU's expedited and +non-expedited grace periods are implemented by the same mechanism. +This means that in the current SRCU implementation, expediting a +future grace period has the side effect of expediting all prior +grace periods that have not yet completed. +(But please note that this is a property of the current implementation, +not necessarily of future implementations.) +In addition, if SRCU has been idle for longer than the interval +specified by the srcutree.exp_holdoff kernel boot parameter +(25 microseconds by default), +and if a synchronize_srcu() invocation ends this idle period, +that invocation will be automatically expedited. +

As of v4.12, SRCU's callbacks are maintained per-CPU, eliminating a locking bottleneck present in prior kernel versions.