From 8203d6d0ee784cfb2ebf89053f7fe399abc867d7 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sun, 2 Aug 2015 13:53:17 -0700 Subject: rcu: Use single-stage IPI algorithm for RCU expedited grace period The current preemptible-RCU expedited grace-period algorithm invokes synchronize_sched_expedited() to enqueue all tasks currently running in a preemptible-RCU read-side critical section, then waits for all the ->blkd_tasks lists to drain. This works, but results in both an IPI and a double context switch even on CPUs that do not happen to be running in a preemptible RCU read-side critical section. This commit implements a new algorithm that causes less OS jitter. This new algorithm IPIs all online CPUs that are not idle (from an RCU perspective), but refrains from self-IPIs. If a CPU receiving this IPI is not in a preemptible RCU read-side critical section (or is just now exiting one), it pushes quiescence up the rcu_node tree, otherwise, it sets a flag that will be handled by the upcoming outermost rcu_read_unlock(), which will then push quiescence up the tree. The expedited grace period must of course wait on any pre-existing blocked readers, and newly blocked readers must be queued carefully based on the state of both the normal and the expedited grace periods. This new queueing approach also avoids the need to update boost state, courtesy of the fact that blocked tasks are no longer ever migrated to the root rcu_node structure. Signed-off-by: Paul E. McKenney --- include/linux/sched.h | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) (limited to 'include/linux/sched.h') diff --git a/include/linux/sched.h b/include/linux/sched.h index a4ab9daa387c..7fa8c4d372e7 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1330,10 +1330,12 @@ struct sched_dl_entity { union rcu_special { struct { - bool blocked; - bool need_qs; - } b; - short s; + u8 blocked; + u8 need_qs; + u8 exp_need_qs; + u8 pad; /* Otherwise the compiler can store garbage here. */ + } b; /* Bits. */ + u32 s; /* Set of bits. */ }; struct rcu_node; -- cgit v1.2.1