rcu: Direct algorithmic SRCU implementation

The current implementation of synchronize_srcu_expedited() can cause
severe OS jitter due to its use of synchronize_sched(), which in turn
invokes try_stop_cpus(), which causes each CPU to be sent an IPI.
This can result in severe performance degradation for real-time workloads
and especially for short-interation-length HPC workloads.  Furthermore,
because only one instance of try_stop_cpus() can be making forward progress
at a given time, only one instance of synchronize_srcu_expedited() can
make forward progress at a time, even if they are all operating on
distinct srcu_struct structures.

This commit, inspired by an earlier implementation by Peter Zijlstra
(https://lkml.org/lkml/2012/1/31/211) and by further offline discussions,
takes a strictly algorithmic bits-in-memory approach.  This has the
disadvantage of requiring one explicit memory-barrier instruction in
each of srcu_read_lock() and srcu_read_unlock(), but on the other hand
completely dispenses with OS jitter and furthermore allows SRCU to be
used freely by CPUs that RCU believes to be idle or offline.

The update-side implementation handles the single read-side memory
barrier by rechecking the per-CPU counters after summing them and
by running through the update-side state machine twice.

This implementation has passed moderate rcutorture testing on both
x86 and Power.  Also updated to use this_cpu_ptr() instead of per_cpu_ptr(),
as suggested by Peter Zijlstra.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index d3d5fa54f25e..a478c8eb8479 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -31,13 +31,19 @@
 #include <linux/rcupdate.h>
 
 struct srcu_struct_array {
-	int c[2];
+	unsigned long c[2];
 };
 
+/* Bit definitions for field ->c above and ->snap below. */
+#define SRCU_USAGE_BITS		2
+#define SRCU_REF_MASK		(ULONG_MAX >> SRCU_USAGE_BITS)
+#define SRCU_USAGE_COUNT	(SRCU_REF_MASK + 1)
+
 struct srcu_struct {
-	int completed;
+	unsigned completed;
 	struct srcu_struct_array __percpu *per_cpu_ref;
 	struct mutex mutex;
+	unsigned long snap[NR_CPUS];
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map dep_map;
 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */