rcu: Add fastpath bypassing funnel locking

In the common case, there will be only one expedited grace period in
the system at a given time, in which case it is not helpful to use
funnel locking.  This commit therefore adds a fastpath that bypasses
funnel locking when the root ->exp_funnel_mutex is not held.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
index 08651da15448e27f8f2fb574c7cd51cde67815c6..97f17e9decdaeced345689e9eb410a79f642b10d 100644 (file)
--- a/Documentation/RCU/trace.txt
+++ b/Documentation/RCU/trace.txt
@@ -237,42 +237,26 @@ o "ktl" is the low-order 16 bits (in hexadecimal) of the count of
 
 The output of "cat rcu/rcu_preempt/rcuexp" looks as follows:
 
-s=21872 d=21872 w=0 tf=0 wd1=0 wd2=0 n=0 sc=21872 dt=21872 dl=0 dx=21872
+s=21872 wd0=0 wd1=0 wd2=0 wd3=5 n=0 enq=0 sc=21872
 
 These fields are as follows:
 
-o      "s" is the starting sequence number.
+o      "s" is the sequence number, with an odd number indicating that
+       an expedited grace period is in progress.
 
-o      "d" is the ending sequence number.  When the starting and ending
-       numbers differ, there is an expedited grace period in progress.
-
-o      "w" is the number of times that the sequence numbers have been
-       in danger of wrapping.
-
-o      "tf" is the number of times that contention has resulted in a
-       failure to begin an expedited grace period.
-
-o      "wd1" and "wd2" are the number of times that an attempt to
-       start an expedited grace period found that someone else had
-       completed an expedited grace period that satisfies the
+o      "wd0", "wd1", "wd2", and "wd3" are the number of times that an
+       attempt to start an expedited grace period found that someone
+       else had completed an expedited grace period that satisfies the
        attempted request.  "Our work is done."
 
-o      "n" is number of times that contention was so great that
-       the request was demoted from an expedited grace period to
-       a normal grace period.
+o      "n" is number of times that a concurrent CPU-hotplug operation
+       forced a fallback to a normal grace period.
+
+o      "enq" is the number of quiescent states still outstanding.
 
 o      "sc" is the number of times that the attempt to start a
        new expedited grace period succeeded.
 
-o      "dt" is the number of times that we attempted to update
-       the "d" counter.
-
-o      "dl" is the number of times that we failed to update the "d"
-       counter.
-
-o      "dx" is the number of times that we succeeded in updating
-       the "d" counter.
-
 
 The output of "cat rcu/rcu_preempt/rcugp" looks as follows:
 

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f66f6e7730bcad099898088bba99263fdeb8307e..3af0dee2d045c6373c6496ef79a6b0367d30a970 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3355,6 +3355,22 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
        struct rcu_node *rnp0;
        struct rcu_node *rnp1 = NULL;
 
+       /*
+        * First try directly acquiring the root lock in order to reduce
+        * latency in the common case where expedited grace periods are
+        * rare.  We check mutex_is_locked() to avoid pathological levels of
+        * memory contention on ->exp_funnel_mutex in the heavy-load case.
+        */
+       rnp0 = rcu_get_root(rsp);
+       if (!mutex_is_locked(&rnp0->exp_funnel_mutex)) {
+               if (mutex_trylock(&rnp0->exp_funnel_mutex)) {
+                       if (sync_exp_work_done(rsp, rnp0, NULL,
+                                              &rsp->expedited_workdone0, s))
+                               return NULL;
+                       return rnp0;
+               }
+       }
+
        /*
         * Each pass through the following loop works its way
         * up the rcu_node tree, returning if others have done the

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 543ba726396c96e1b6cb5b4fe01582e051ec2ede..80d974df0ea02cedc1bf944f54204e8e83dc999d 100644 (file)
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -493,7 +493,7 @@ struct rcu_state {
        /* End of fields guarded by barrier_mutex. */
 
        unsigned long expedited_sequence;       /* Take a ticket. */
-       atomic_long_t expedited_tryfail;        /* # acquisition failures. */
+       atomic_long_t expedited_workdone0;      /* # done by others #0. */
        atomic_long_t expedited_workdone1;      /* # done by others #1. */
        atomic_long_t expedited_workdone2;      /* # done by others #2. */
        atomic_long_t expedited_workdone3;      /* # done by others #3. */

diff --git a/kernel/rcu/tree_trace.c b/kernel/rcu/tree_trace.c
index ec62369f1b0243ad91a0cf978a0bd914d2f8c810..6fc4c5ff3bb5cf55925a1da6ba73d269d3349d49 100644 (file)
--- a/kernel/rcu/tree_trace.c
+++ b/kernel/rcu/tree_trace.c
@@ -185,9 +185,9 @@ static int show_rcuexp(struct seq_file *m, void *v)
 {
        struct rcu_state *rsp = (struct rcu_state *)m->private;
 
-       seq_printf(m, "t=%lu tf=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n",
+       seq_printf(m, "s=%lu wd0=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n",
                   rsp->expedited_sequence,
-                  atomic_long_read(&rsp->expedited_tryfail),
+                  atomic_long_read(&rsp->expedited_workdone0),
                   atomic_long_read(&rsp->expedited_workdone1),
                   atomic_long_read(&rsp->expedited_workdone2),
                   atomic_long_read(&rsp->expedited_workdone3),