rcu: Don't make callbacks go through second full grace period

RCU's current CPU-offline code path dumps all of the outgoing CPU's
callbacks onto the RCU_NEXT_TAIL portion of the surviving CPU's
callback list.  This means that all the ready-to-invoke callbacks from
the outgoing CPU must wait for another full RCU grace period.  This was
just fine when CPU-hotplug events were rare, but there is increasing
evidence that users are planning to make increasing use of CPU hotplug.

Therefore, this commit changes the callback-dumping procedure so that
callbacks that are ready to invoke are moved to the RCU_DONE_TAIL
portion of the surviving CPU's callback list.  This avoids running
these callbacks through a second unnecessary grace period.

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

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index ac3a810..7789e66 100644 (file)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1270,24 +1270,64 @@ static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
        struct rcu_data *receive_rdp = per_cpu_ptr(rsp->rda, receive_cpu);
        struct rcu_node *rnp = rdp->mynode; /* For dying CPU. */
 
-       /* Move callbacks to some other CPU. */
+       /* First, adjust the counts. */
+       if (rdp->nxtlist != NULL) {
+               receive_rdp->qlen_lazy += rdp->qlen_lazy;
+               receive_rdp->qlen += rdp->qlen;
+               rdp->qlen_lazy = 0;
+               rdp->qlen = 0;
+       }
+
+       /*
+        * Next, move ready-to-invoke callbacks to be invoked on some
+        * other CPU.  These will not be required to pass through another
+        * grace period:  They are done, regardless of CPU.
+        */
+       if (rdp->nxtlist != NULL &&
+           rdp->nxttail[RCU_DONE_TAIL] != &rdp->nxtlist) {
+               struct rcu_head *oldhead;
+               struct rcu_head **oldtail;
+               struct rcu_head **newtail;
+
+               oldhead = rdp->nxtlist;
+               oldtail = receive_rdp->nxttail[RCU_DONE_TAIL];
+               rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
+               *rdp->nxttail[RCU_DONE_TAIL] = *oldtail;
+               *receive_rdp->nxttail[RCU_DONE_TAIL] = oldhead;
+               newtail = rdp->nxttail[RCU_DONE_TAIL];
+               for (i = RCU_DONE_TAIL; i < RCU_NEXT_SIZE; i++) {
+                       if (receive_rdp->nxttail[i] == oldtail)
+                               receive_rdp->nxttail[i] = newtail;
+                       if (rdp->nxttail[i] == newtail)
+                               rdp->nxttail[i] = &rdp->nxtlist;
+               }
+       }
+
+       /*
+        * Finally, put the rest of the callbacks at the end of the list.
+        * The ones that made it partway through get to start over:  We
+        * cannot assume that grace periods are synchronized across CPUs.
+        * (We could splice RCU_WAIT_TAIL into RCU_NEXT_READY_TAIL, but
+        * this does not seem compelling.  Not yet, anyway.)
+        */
        if (rdp->nxtlist != NULL) {
                *receive_rdp->nxttail[RCU_NEXT_TAIL] = rdp->nxtlist;
                receive_rdp->nxttail[RCU_NEXT_TAIL] =
                                rdp->nxttail[RCU_NEXT_TAIL];
-               receive_rdp->qlen_lazy += rdp->qlen_lazy;
-               receive_rdp->qlen += rdp->qlen;
                receive_rdp->n_cbs_adopted += rdp->qlen;
                rdp->n_cbs_orphaned += rdp->qlen;
 
                rdp->nxtlist = NULL;
                for (i = 0; i < RCU_NEXT_SIZE; i++)
                        rdp->nxttail[i] = &rdp->nxtlist;
-               rdp->qlen_lazy = 0;
-               rdp->qlen = 0;
        }
 
-       /* Record a quiescent state for the dying CPU. */
+       /*
+        * Record a quiescent state for the dying CPU.  This is safe
+        * only because we have already cleared out the callbacks.
+        * (Otherwise, the RCU core might try to schedule the invocation
+        * of callbacks on this now-offline CPU, which would be bad.)
+        */
        mask = rdp->grpmask;    /* rnp->grplo is constant. */
        trace_rcu_grace_period(rsp->name,
                               rnp->gpnum + 1 - !!(rnp->qsmask & mask),